Questions seem to come in spurts. Most I can answer with confidence, particularly if it's a subject I can research before offering some type of advice. This time, well... I'm at a loss.
We were sitting as a family watching a tv show that included a sex scene with two guys. It didn't really show anything but it had an obvious effect on our son. I mean, a serious log snaked down a pantleg. He did his best to discreetly adjust but that's actually what caused us to notice.
Now we don't care if he's gay or not we just want him to be happy and well adjusted. How do we handle this? Do I just come out and ask him if he's gay?"
Well, the boner doesn't mean he's gay. At his age, all guys are super-charged with hormones and pretty much any sex scene could cause a hard-on. Hell, I'm sure you remember at that age your cock didn't need any reason to pop wood. Without notice, wham!
There it is: Hard as a tree limb. And, damned uncomfortable if crammed into pants.
But do you ask him if he's gay? I don't know. I've got zero experience with this. My inclination is to say, "No. Let him come to you." But, at the same time, he'll need your support, to know you love him and don't care if he's into cock.