Have you tried making it into a big game and getting him to climb in himself? My 2 year old loves climbing in himself and thinks its great fun. I strap him in quick when he's still laughing from his achievement and before he's realised we're off!
Or you could try the sticker chart thing. Everytime he does something you ask give him a sticker to put on his chart. When he has 3 stickers give him a treat. That way you can then say if he gets in the car he can have a sticker for his chart.
I can't answer this one - but if someone else can answer a related issue. My four year old unhooks the latch on her car seat and gets out of the seat while driving. I have tried three different seats and nothing keeps her hands unable to undo.
