43 y/o here.
Spent 2 years on campus, 1 year off, 2 final in the fraternity.
I LOVED my fraternity experience. Had much more fun than my dorm years - by far. But I don't know that I would've felt the same way had I joined the fraternity immediately. And my fraternity was new on campus at the time, and very easy going. None of the hazing or general bs you typically hear about fraternities.
As for grades, my best grades over time were probably in the dorms, although my worst and then my best semesters were in the fraternity. But obviously that's more about the person and how they handle their environment. Some thrive, some may become too distracted by all the opportunities. I'm not sure I would worry much about one over the other. And it does come down to how much the particular fraternity handles grades. Our 1st semester back on campus, our house GPA was pathetic, and our Chapter advisor really laid into us - and big time. And it made a difference. Other houses may not care as much. If you have genuine concerns - most houses have/had chapter and faculty advisors. You may want to seek those people out.
I would say it probably comes down to how your son really feels about the fraternity itself. Does he feel comfortable with the guys in it. Does he like their house and amenities? I know too many kids who just jumped into a house so they could be in a frat, only to hate it. My path was a bit more fortunate - I got to know a lot of the guys in the hosue before joining it. The best thing is you get to have a bit of a choice going into the fraternity. You don't really get that moving into a dorm.
I didn't NOT enjoy my dorm experience (mostly). But to me it was much more of an every man for himself type of arrangement. There wasn't much cohesiveness, and living is such close quarters with people you likely don't know, perhaps don't like - can get tiresome. What was worst was living in Towers my freshman year and having I don't know how many fire alarms every week. Got old really fast.