OP, how old is your son? At 22 kg he might be getting to an age where something like a Trail-a-Bike would be appropriate. He's not likely to get more than a summer's riding out of a trailer, I wouldn't think.
A hybrid with decent wheels should be able to support you and him (this summer) if he's too young to sit up by himself.
And don't worry about the kid with a helmet on. There are questions about helmet effectiveness, true, but the fall-over-to-the-side accident is what bike helmets are designed and tested to. Not that the boy would appreciate that kind of accident!