I'm surprised from what I've read here that this hasn't surfaced before.
In case it's any value, let me see if I can explain some of it from the male perspective.
There's this whole "male bonding" thing that exists for many men. Although it's not something I've ever had need for in my life to any degree personally, I've seen it A LOT with other friends - including one of my own sons. Not going to try to analyze it here as it's pointless. Enough to acknowledge it's existence - and importance for some men. I think some women experience it too, but it seems to be in smaller numbers.
I may even go out on a limb and credit a certain percentage of it to maturity - or lack of. Probably get spanked for that but it is what it is !
Now, it's a bit of a growth process for guys that seem to need this. At some point, their life seem to fill (or not) with other things that relistically need to take priority. Maybe wives/GFs, children, job responsibility, changes in interests etc. And it passes. But not without pain it seems. There's a period there where they feel they are being pulled in multiple directions and what does THAT cause ? Pressure! So it can be an turbulent time, emotions can become unstable, thinking muddy and decisions.........well........unpredictable.
I always relate it to some of those growing up phases that we all hit at various times in our lives. Somehow we (most) seem to get past them. Like milestones.
And of course, the "group" depending on their levels of maturity, lifestyles etc don't want to let go ! So there's quite a lot of pressure on anyone who seems to be prioritizing otherwise. It's ugly - really. Borders on Jr High level of peer pressure
So, question is - what's your 'sensible' and prudent reaction ? I suspect you know
You have to trust his ability to reason clearly, empathasize with the position he's in, try to be supporting and loving. How you do this depends on his personality. I like to try to keep things light - bordering on humorous. But that can exacerbate issues with people sensitive to it.
And I wouldn't hesitate to express to him that you 'know' the dynamic, that these guys have NO respect or love for you, and that you trust him to not let them grind you into the carpet when you can't be around to defend yourself. Only simple respect - right ? That 'should' be nuff said ?
Beyond that, you can't handle the relationship like you would a child. He's a big boy. If he's going to be that easily influenced by other people in his life, you may as well find it out now and confront it. And HE needs it as part of his growing up process. It's all about making decisions & setting priorities in our lives. This is a bridge he has to cross eventually. In general, the sooner the better. For you all.
Good luck and keep us posted !