K.H.
I think he may be a little to young still , I don't think you can even get helmets for that young a child. You'd be best going to a local bike shop and talking to them , I see a lot of people on the cycle track with babies in the pull along cars rather than on a seat on the back of the bike , he may need to be around 2 before he can go on the back of your bike.
Hope this helps.