It will not turn into either I am afraid, but that arguably makes it more interesting for yourself and your son. This larvae will turn into a Sawfly, which is a type of wasp. It looks like one of the cimbicid group (Cimbicidae) and one of the Trichiosoma species, possibly Trichiosoma lucorum.
I have never found this species and they are not common. Try feeding it on Birch leaves, and when fully grown (it may be already) it will spin a cocoon and pupate within in. All being well it will emerge next summer if kept in a cool shed or garage over winter.