I just want to add that in my own recovery I sometimes had a hard time with the "cheerleading".
I found it really annoying because I was in early recovery and "nobody in my family knew what I was going through" in fact I held a lot of anger and blame for my family at that point and I don't think they had any idea how much was brewing under my surface, some really great pink cloud stuff, but still a land mine of resentments...and I hate to say it but parents are often the mother lode of resentments for addicts/alcoholics. Deserved or undeserved...doesn't matter!
The rah rah could just be striking a nerve. And the more upbeat you try to be I bet the more sulky he becomes. I bet he's not being sulky with the lads in the house, because they would call him on his BS in a heart beat...or if it isn't BS but an actual issue then they will get in there and explore it in the steps and deal with it.
Cheeriness, trying to "cheer him up" is probably just not what he needs. Let him be with the guys...and just don't let him sniff your fear out and burden you with the sulkies. Nor burden him with your requirement that he be cheerful right now in order for you to feel okay.
He's in a great situation to grab hold. And he is a young man. Let him be.