Ah the Joie seats are pretty notorious for this, because the pads are so bulky, it creates a much bigger gap at the tummy and so children (esp if they are slim) can suck in their tummy and create a really big gap to get their arms through. You could try putting some biggish gummy wristbands on him - e.g. those charity wristbands or spare straps for fitness band (watches). However he might just take them off.
The headrest also has to be much higher than you'd expect in this seat to be at the right height - you go by the position of the straps rather than where the headrest looks right. For rear facing the straps should be level or just below the shoulders, for forward facing level or just above. Depending on his understanding you could try making it slightly less tight as a sort of "deal" for him to keep his arms in. Obviously it still needs to be snug to his body but it's OK if you can fit two fingers flat under the straps. Sometimes people think you need them as tight as humanly possible to the point it's uncomfortable for the child.
You could also try a reward chart so if he keeps his arms in all the way he gets a small reward leading up to a bigger one to help break the habit. Or a consequence if he takes his arms out although it's tricky at 2 because it needs to be fairly immediate and there aren't many things which will work when out and about.
In terms of physically preventing it, you could try Five Point Plus - it's crash tested and doesn't affect the workings of the harness so is a bit better than the clips which hold it together. It just blocks the area of the straps where they can make a gap. https://5pointplus.com/ Or if you prefer to try a clip, Besafe Belt Collector is a good one as it is designed to come off if there is an accident. I would use that preferably over the Houdini stop or random non branded ones.
If no luck, you might want to consider one of the Cybex Pallas impact shield seats. They have a different design so no straps and can be harder to escape. They tend to work well in vans but if you don't have isofix then you'll need to find one of the R44 ones which fit with seatbelt only.
There are other seats from other brands which are belt fitted but the padding on the straps is less bulky. Forward facing you could try Britax King Pro or rear facing the Axkid seats. However this is a bit of an expensive solution and if he already knows how to escape a 5 point harness it could be he does it in any 5 point harness seat.