Get him checked out thoroughly. It may well not be fetal alcohol syndrome, and it would put your mind at rest, knowing one way or the other.
There are many other causes it could be.
If he has mental issues you need to know what they really are, and get him the appropriate help.
You can talk to the doctor about that rather than declare you think he has fetal alcohol syndrome straight up.
My understanding is that fetal alcohol syndrome usually results from constant highly excessive drinking during pregnancy.
If you don't get it investigated you are likely to torture yourself for the rest of your life.
Or fail to discipline him properly and create the same issues as if he really did have fetal alcohol syndrome.
Boys particularly can go through periods of behaving like complete dumb idiots, particularly at that age.
Okay, so you feel bad. Take a moment to feel what you're feeling, then pull up your boot straps and get on with it. First step is writing your concerns down. Be as specific as possible. Then book in a GP appointment. Tell your GP what's going on and get a referral to a paediatrician to have your boy assessed.
It may be nothing. Or it may not.
But no amount of worrying alone will help.
It wasn't that long ago when women were recommended to drink durng pregnancy (a Guinness a day for iron). And before that, there was no guidelines for drinking anyway. People would still carry on as normal.
You're not Satan incarnate. You were young, and you made a mistake. It's time to atone for that by doing what needs to be done for your child now.