I think you should look past your brother's minor faults and not try to label him with a mental disorder.

I don't understand how you go from he forgets groceries to he might be dumb enough to get someone pregnant by accident..like he's smart enough to get into college but not pass sex ed. Or maybe I just fail at understanding your post. I didn't understand the "take care of the family" part.. he's 17..who does he need to take care of besides himself? Sounds like pressure for something.

Also.. could be other medical reasons for being forgetful. For instance, people with thyroid issues have problems remembering things when their medication is off. Stress can cause someone to go shopping and forget what they went for.

If he's one of those computer/video game people..they're known for being in their "own little worlds".