How do they keep normal people from jimmying the anklet with a screwdriver? By making it tamper-evident. Sure, it's possible to compromise the electronics, but you're going to have a lot of explaining to do when they take a look at it.

So the basic idea is that if Ant-Man shrinks out of the bracelet, it either sets off an alarm or is indelibly marked (or both). Perhaps an elastic band that, if detached, will shrink and bring electric contacts together, triggering an alarm. Or the contacts could be latches that will break if forced apart.

Use an ankle monitor that also records Ant Man's pulse

If his pulse stops, then it sets off an alarm, indicating that he's either dead, or he has removed it (probably by shrinking).

In general, the fact that ankle bracelets are hard to remove, is really only secondary to the fact that if they are removed the authorities are notified, or at least that the tampering is evident. I mean, almost anyone can get out of an ankle bracelet with a pair of bolt cutters anyway.

House arrest is used when the offender is actually trusted by law enforcement to comply with the terms of their house arrest. Violating those terms is grounds for the privilege of house arrest to be revoked, so it is usually not worth it to them to violate the terms. The same philosophy would apply to Ant Man.

Shrink his house

Give him a taste of his own poison. Shrink his home and have it kept inside a glass dome. This makes it easier and cheaper to have surveylance 24/7 around the house, so that if he wants to leave, he'll have to traverse the quantum realm. That's opening up a storageroom full of cans of worms, so it might just be deterrent enough.

The house is surrounded by a moat. A small one, filled with vinegar. Humans just step over. But an real-life ant could not traverse it. Some ants can swim or float in water. The purpose of the vinegar is to make it too noxious to try. Or use water with some oil on top. This reduces the surface tension of the water so that the ant can not float.

Ant Man is an ant-sized human, not an actual ant, but he'd still need to either float or swim across the moat. Make the sides too slippery to climb up and the liquid impossible to survive.

So maybe Ant Man shrinks to exit the house then grows to step over the moat then shrinks again to sneak away. To prevent that, make sure the moat has a fence on both sides. No human can get in or out of it and it's too small to transform to human-sized while inside. The fence can be electrified so the small version of Ant Man can not climb it.

Honestly, room arrest (in a lovely glass or plexiglass cage with mini-moats at the threshold of the door and any other openings) makes much more sense. Combine it with great security cameras to know when or if he shrinks. Not fun for privacy, but necessary due to his superpowers.

If he can stay shrunk down for the duration of his confinement, then your job is easy. Constructing a box with tiny doll furniture and no escape is quite simple. The boxes probably already exist retail.

If you can't shrink him or confine him to a room or set of rooms, your best hope might be psychological. Make the penalty for violating the house arrest too great to transgress. Like custody of his kids, if he has any. Losing his job. Etc. Fines wouldn't do it as whoever breaks him out would just pay them.

House arrest in real life is only sometimes literal. In the United States, prisoners are allowed to leave their homes for work, school, medical and other appointments, even to run errands. It's restricted and they have to inform the police where they are going and when. Their ankle monitors are to find them if they flee.

When KGB was monitoring someone and wanted to be sure to track them wherever they'd go, they used to sprinkle some radioactive dust on the door mat.

Then a Geiger counter was sufficient to their movement, since the radioactive dust, sticking on the sole of their shoes, would emit radioactivity all around.

The only way to go around this was to strip naked and throw away all one was wearing and replace it with new things (CIA knew about this trick after losing some agents).

Antman can shrink, but shrinking won't remove radioactive dust sticking to his clothes/shoes. Just set a Geiger barrier around the confinement area, and as soon as it triggers you know that he is trying something.

what would be better methods to enforce house arrest on a shrinking man?

Use people

You are incarcerating a superhero. That warrants the presence and expense of a fulltime guard. Details like privacy and comfort can be negotiated later, but since this is an alternative to prison, I think Ant Man would be willing to sign away the privacy considerations as part of the terms.

Of course, the guard could be incapacitated, but the goal isn't to keep Ant Man confined to the home, but rather detect if he gets out, then send an army to collect him. If a guard, consisting of 2-3 people at at time, loses visual for more than five minutes or whatever, it can be reported that he has left the premise.

A back of the envelope calculation for cost would be three guards at once, three shifts a day for two years, or about twenty salaries. If these are extremely competent and highly-capable guards, we're looking at an order of magnitude $100K salaries or an upper-bound of $2 million total to keep Ant Man under house arrest with full confidence that he didn't leave (or immediately detecting if he does).

Insecticide

Have heat sensors in every room. If he shrinks they will trip and set off insecticide spray.

Look at this https://www.asianscientist.com/2018/05/in-the-lab/shape-memory-porous-material/. It's a metal fibre that changes its shape depending on the prevailing conditions , it gets smaller the hotter it is and larger the cooler it is. If ant man shrinks , then he is reducing his mass , by the famous $E=mc^2$, formula for every bit of mass he loses energy is lost to the environment. This energy loss will heat the surroundings, so if this metal was placed around his ankle and he started to shrink it would also shrink with him. Now it should be noted that there is a minimum size to which this metal can shrink , but then again if you arranged it instead in some type of coil rather than just a normal circle , that the coil would pull closer together as it shrunk so it could reach a still smaller size relative to its starting shape.

I think this is the closest one could have with current technology to an ankle bracelet that can still be applied to a shrinking man.