It's awkward, of course, to think about your kid pumping away at an appliance in the next room-and, hey, here's hoping he doesn't leave it in the sink after cleaning it. But if you want your son to arrive at partnered sex without a bad case of death grip syndrome, a Fleshlight is a safer option than a fist.
And boys, unlike girls, usually don't need to be encouraged to masturbate. If you would get your daughter an insertion toy-one that vibrates, something flesh-and-blood dicks can't do-why would you hesitate to get your son an insertion toy? Yes, boys have hands (so do girls) and boys can make fists.