If he's over 2.5 he should be old enough to use reason of sorts. You could try this: Line up a few days out (things he really wants to do). Explain to him about safety etc and how he must never ever take his arms out because the seat bet won't work properly. Tell him that if he takes his arms out, you will turn around and go straight home again. Get ready to go on your trips out. If he takes his arms out, tell him that because he is not putting his arms in his seat belt, you are going to go home again. Do this no matter what. Remind him until the next trip out why you went home. Do the same thing next time, reminding him to keep his arms in. (I would really try not to take him on trips he doesn't like during this phase, as he'll just use the arms to make you go home again). Repeat until he gets the message.
He should be upset enough about missing his nice things for you to be able to start using the threat of turning round and going home again for any trip, even ones he doesn't like. A sort of Pavlovian conditioning, if you will.
If he is over 6 though, you may have to use a modified version of this involving withholding treats, as he would otherwise manipulate the system for his own ends.