Should a 3.5 year old be able to dress himself?

I have a 3.5 year old and he can't dress himself. He asks for help every time and when I ask him to do it or tell him to try he won't. Even when he goes to the bathroom he sits on the toilet and yells for me to come help him pull his underwear and pants on. It's driving me bananas because I feel as though he CAN do it he just won't. Also, I have a 9 month old and he seems to always go to the bathroom when I am putting her down for a nap which makes it difficult. Should he be able to do it yet? Or at least try to do it? I've bribed, reverse psychology everything, maybe he really just can't yet?