I think airfix is a bit tricky, but yes, I don't see why not. 7 year old boys that I look after certainly like to make things. If you've got a Hobby craft near you, you'll find tons of stuff there to chose from.
My kids are that age/gender!!!
They both love crafty things,games, music, books and pyjamas!!
DS - bionicles
models (the gluing sticking kind!!!)
DD Hair stuff
Littlest Pet Shop
I think something crafty is probably a very safe bet though