sprichst du Deutsch also?
Like someone said before, he probably hasn't developed enough yet to know his body and his sex drive...
Hell, at seventeen I was quite active in the field
If it's really what he believes, then he should get his girlfriend used to that, and she'll either accept that or run for the hills.
Else, someone should really teach him that there's nothing "dirty" or "wrong" about healthy safe sex, let alone a handjob. He masturbates, I presume, so he's already had countless of them.