I have never asked. I have made comments about them getting drunk with my friends and I. I am not bothered but sometimes think it may mean he doesn't take our relationship seriously.
Perhaps the situation has never come up. If he was against you meeting his friends i.e. a situation arose where he avoided you meeting his friends, then there might be cause for worry. I know with my OH, his friends just don't do the kind of stuff/go the kind of places where our paths would cross or they'd be interested in coming along.
As you said about getting drunk with his friends, it may be that his friends aren't big drinkers or frequent different pubs and clubs to the ones you'd tend to go to.
I wouldn't read too much into the issue. If he didnt take the relationship seriously, I dont think you'd have been together for so long without this being the only issue.