@tee95,
You both are young, were what 19 and a half or 20 when you got together. Boys will be boys, there is a sense of power knowing that some girl thinks he is hot, that's why he said to her he was nervous. You have no idea what he would have done from there in reality as it didn't get to the point where that presented itself. But, if two people have chemistry you can probably bet something would have happened.
Having said that, I had a talk with my nephew a couple of years ago, he was at a cross road. He loved his girlfriend, but they two got together young and he's in a band. He wanted to play the field but didn't want to lose her and was quite lost in himself. We had a chat and 2 years later they are still together, he decided to stick with "love"...
He may be in a committed relationship tee, but you have to keep that committed relationship alive, with adventure, passion as well as laughter and love. Even at your age.
Perhaps the 20 days apart led him to go out more. You state that you've tried to get him to socialise more. He did, he had drunk friends, she had drunk friends, they appeared not to be drunk, consequently got chatting and you weren't there.
So you need to also consider if he just stays at home because that's what he feels you want. Maybe he needs to get out there in the big wide world but loves you and is at a cross road like my nephew.