He is in love. Your post just described most of the symptoms.
Oh, but they are. At least in part. This is not an attack - you and your other kids have a right to feel comfortable in your home. And it’s not meant to dismiss your concerns about his mental health, which I’m sure are valid.
But he’s 20. Still a family member, but not a child. His mental health and his relationships are your worry, but not your business any more, unless they impact you directly. So you have a right to ask for polite interaction, no blowing up in your face because you have offered him breakfast, and of course the usual chores you have always expected from family members, whatever they are in your house. I mean, you could ask for rent if he makes money from his internship, but I personally wouldn’t unless it is necessary for your financial comfort.
I do think that you have a right to point out that the texting and FaceTiming during the workday can get him in trouble with his employer because it’s part of his education and that is still your business. But not his love life.
I agree his choices sound immature and possibly damaging, and all around he’s acting more like a 14 yo than a 20yo, but it seems that is part of his mildly neurodiverse make-up. His social and sexual development hasn’t caught up with his legal age yet.
It’s a huge dilemma. But I agree with the posters that the most important thing is to keep the channels open so you can tell when proper derailment (losing jobs, failing classes as opposed to dropping friends and ECs) is happening and try to provide a soft landing in case he comes crashing down to earth. Which may or may not happen. Let them grow up a little.