I don't think I've ever used anything heavier than worsted for a baby blanket. One hint my mom has used is to choose a pattern that doesn't have too many holes a baby can get his/her fingers tangled up in. Washable is important.
I'd say choose a worsted weight yarn in a boy-color (like a denim blue). That makes it pretty for you to work, and still something a little boy might be willing to carry around.