It wouldnt stay erect during oral stimulation. It kept going soft in between. And even as he came, it wasnt fully hard. Is this a sign of ED? This isnt the first time this has happened. He wasnt drunk or anything.
This happened several times before and he's 19 if that helps.

He didn't didn't feel comfortable and the reason could be anything.

He might not feel confident to know that you're looking at his penis. He might not like seeing you going down on him for some reason.

What's for sure is that he was thinking about something that distracted him.

If this was a one time occurance, he either wasn't really into it or his mind was elsewhere (I guess those are sort of the same thing).

One word....v.i.a.g.r.a