In the devil's advocate position, the violent incident that hospitalised him might not even correlate to Summer's absence, it could simply be he got into serious brawling as his way of dealing with life, maybe crossed with the remnants of the gangs mentioned in the original WIAB. But my bet is the violent incident is tied in.
I also think that the violent incident that landed the MC in hospital may be unrelated to Summer. We're not given any context for it; Nami just mentions it and then the MC has the flashbacks from which we learn that he was seriously injured, but there is nothing to indicate how or when it happened.
During the sessions with Amber we find out that the MC would frequently get into fights at school as a way of coping with his anger. That hasn't really changed since then. I mean...
-the fight with Sai and his buddy where the MC gets punched
-almost getting into a 3vs1 fight with Sai and two other guys; Jeff steps in so nothing happens
-acting provocatively with Dwight and his crew. Under different circumstances this could have escalated very quickly
-violently pushing Justin for no good reason during the talk with Nadia (just before the tabletop session)
It's not implausible that one time he just messed with the wrong guy and things took a really bad turn for him. If that's what happened, he clearly hasn't learned his lesson, lol.
I have yet to play WIAB so I can't comment on the gang suggestion, but it's been mentioned before so it's worth keeping in mind.