It's explained better/in more detail in the course but -
The positive opposite to getting out of his seat is staying in the seat properly with the straps on his shoulders.
The positive opposite for hurting the baby is some kind of gentler play e.g. tickling the baby's feet, showing them a toy etc. (Of course, not in the car, but if you can get him to stay in his seat then you might not need to work on "not hurting baby in the car" because he won't be able to reach, hopefully.)
A good way to think about it is basically like what you'd do for potty training - you can do this over literally any behaviour issue. (It's a good analogy too because of course nobody would potty train by shouting at the child when they have an accident! You know that you also have to train them in what TO do).
The first tool you learn in the course is "special praise" (which sounds a bit wanky I realise but it does seem to be total gold currency with children this age) - so every time you put him in the seat, you'd go totally OTT amazing "Wow!!! Look at you! You've got your seatbelt on!! What a good boy!" etc. Even though he's only had it on for three seconds. It doesn't matter - what you're looking to do is give him the message: I REALLY REALLY like it when you have your seatbelt on. That's good. That's the behaviour I want to see. Then any time you look back and see that he has his belts on, you can repeat this special praise again. Like when potty training, if there happen to be any other adults in the car, clue them in on the game as well and get them to be really excited and impressed too (and then just to ignore if he does take the seatbelt off, but obviously you stop the praise and attention at that point, and preferably stop the car as soon as it is safe).
You could also use some of the other tools in the course like practising - bring his car seat into the house if you can safely use it like that (if not, borrow another one if you can, or even make a pretend car seat with "straps" or use a buggy, high chair etc) and tell him you're going to play a game where he sits in the seat, you strap him in, and he has to stay in the seat. Again go overboard with the ridiculous praise. Use challenges "I bet you can't stay in the seat for 1 minute!" then act disbelieving, surprised, excited, when he does stay in the straps. Put another chair so you can pretend to drive, if you like. Let him have any normal distractions he'd be allowed in the car, it's not supposed to be a test of boredom endurance. Or you could do this in your actual car sitting in it parked, or with the engine running but - it just might be more tricky to keep the baby happy while you do this, so maybe only an option when another adult is available.
Another tool is shaping or scaffolding - which means you build up from where you are now to the point you want to be. So it might be time - if he currently tolerates the straps for 1 minute, then you work on 2 minutes, then 5 minutes, then 10 minutes and so on. (You might find once you get to about 15 minutes, then you don't need to train any longer periods). Or if he currently is happy with the straps around his waist but not his shoulders, you could shape by building up - straps around waist only, straps very loose but arms in, a bit tighter, a bit tighter etc. Again it would be best to practice this when the car is not actually moving, because it's not safe to travel with the straps very loose either.
If praise is not enough you could also look at a reward scheme, like a sticker chart or a points chart.
You can combine with mild sanctions, like the loss of a privilege, or a short time out e.g. facing the window with the car stopped, if you have to stop the car to get him strapped in again - these will be more effective when combined with the above positive "training" type behaviours, compared to when you're just trying to use punishment/telling off etc alone.
If you do see him take the seatbelt off it is a good idea to stop the car as soon as possible and strap him back in, or wait until he puts his arms back in himself. You CAN do the praise once he's strapped in again - again this is showing him the contrast between the lack of attention/praise when he takes his arms out, and the OTT attention/praise when you see him with arms in. You won't have to do the OTT praise forever but it is important to do it as much as possible while you're building the habit of the new behaviour, because the current behaviour is currently very reinforced - he gets free, which he probably likes, he gets loads of attention (even if it's negative attention), he gets to hurt the baby which gets him even more attention/power. Try to avoid giving attention beyond the minimum of obviously needing to stop and get him safely secured again.
Once he starts to get the idea that he gets praise/attention/rewarded for keeping the belt on in his car seat, you could also start looking for positive things to say about it e.g. books to read, TV programmes, there are some cartoons on youtube etc (search "safety seat cartoon"). They might say "seatbelt" instead of "straps" (but TBH with my DC we just call it all seatbelt, seems easier and prevents arguments). Make him the "safety officer" in charge of making sure everyone has their seatbelt on etc.