Note: I tend to write long posts. MN have started truncating them so some of this will probably get cut off, if you see a "Read More" button there are more suggestions under there.
At his age if there are no SEN you might be able to use behavioural methods.
ie, make it really positive to wear a seatbelt. Everyone emphasises the safety and goodness/rightness of wearing a seatbelt. Praise him massively when he sits properly in the seat and lets you do it up. Praise massively when you are halfway somewhere and his seatbelt is still done up (if he struggles with getting halfway, try praising 20% into the journey - breaking into v small chunks where he can be successful can help a lot). Talk about seatbelts with other people he likes as well like grandparents, nursery staff, random strangers on the street etc, e.g. if he's done well that morning then gushingly report to his keyworker that he did so well keeping his seatbelt on today. Or as you leave, make sure a keyworker hears you reminding him to keep the seatbelt on today, so that they can back you up and reinforce the message.
Some kind of reward attached to keeping the belt on. A sticker, a reward chart, being able to choose the music or have access to a certain toy, something nice when you get where you're going, whatever might work for him.
Possibly a consequence if he undoes it. Though it would need to be something fairly immediate.
Also, don't forget to make sure it's comfortable. The straps do need to be snug, but they don't need to be so tight they are uncomfortable. Check the headrest is adjusted so it's not squashing his shoulders, and that the pads are on the straps properly so they are soft against his neck. Get him to sit with his bum back and lean back into the seat so he's not being pinned in a crouched or slumped position when you tighten the straps. Pull his clothing through so the straps aren't rubbing his skin, esp in shorts.
You can also make it uncomfortable for him to press the button by sticking the prickly side of velcro to it. That is sometimes enough to deter young children, as they have sensitive fingers.
The praise/reward/social approval/etc are the most effective for most children, but not all children do respond to them. Some children respond to praise by immediately doing the opposite of the thing they are being praised for.
If he doesn't tend to do well with rewards and punishment, you can try approaching it in a way which gives him more control - make a serious thing about how he's worked out how to open the seat, which is very clever and responsible, but if he's going to press the button, it's important to know when to do it and when not to do it, a bit like how he knows (some other safety rule), which he didn't used to know when he was little. You could make him the "safety officer" of the car, since he's getting to be such an expert about seatbelts, maybe he would like to know why we all wear seatbelts and check/remind that everyone else is wearing them. Then (depending on his personality) either teach him the rule that the car has to stopped and parked fully, and how to recognise this, OR that he needs to wait for a signal word from you before he opens it. If he doesn't wait, then mum/dad will open it for him (I know that logically this does not work, but a three year old does not know that. Or you can fasten it and then immediately open it, which usually annoys them sufficiently to make the point.) But essentially, teach him the appropriate time and place to open the button and then give him the opportunity to do it in that context.
You can do this less verbally, e.g. using a pictoral social story to show when the button is OK to be pressed vs when it is not OK to be pressed, and possibly also what happens if the car crashes with no seatbelt on (in the form of e.g. a bumped head and crying face)
Lastly, do make sure the child locks are activated on your rear doors! And ultimately, a seatbelt that he keeps on is safer than a 5 point harness which he won't keep on, but it is worth persevering with the harness as it is much easier for them to lean and stretch out of position with the seatbelt. Once he's over 4 and 18kg the evidence is not so clear that 5 point harness is safer, so it's not terrible if you end up switching.
Good luck!!