I would get him something that he likes.
For Tom's birthday I got him a new fishing pole and some bait... he loved it!
I think your son should get him a water gun (and get one for your son as well) so your hubby and teach your son how to shoot. And you wouldn't have to worry about any accidents as it's only water :)
Does you hubby have a gun magazine that you could look through to get some ideas?