Do you have any ideas how I can encourage my nearly 3 year old son to wear a kippa, and keep it on? We wears it only in gan, and then not all the time. May I "bribe" him with presents of chocolate if he does?

I would suggest not making a big deal of the kippah issue at three. You can give him encouragement, a compliment, even an occasional prize, but it should not be an "issue". If not wearing it is a way to get attention, the best course is to ignore it totally, if it is just uncomfortable, a little encouragement is ok.