S.D.
I agree with PP, if you can't put a helmet on him, it doesn't matter what type of seat or trailer you get, he's too young. Once his head is big enough for a helmet (you'll have to spend more money to get an infant helmet if you don't want to wait until he's at least 1) then look at the guidelines for seats. We got one for only $20 at Once Upon A Child, but it does require that the baby can sit unassisted.