Helping tweens stand up to peers

I need some help from the POOKs out there...

We're blessed that Henry has an active, comfortable and healthy social
life. He has a good group of friends that contains some "regulars" and
then another element that seems to come and go. They hang out at each
other's homes (we make sure parents are there), go to movies on
weekends, maybe meet at Barnes and Noble in the coffee shop, etc. We
always know where he is and we usually know the parents of the kids he
is with. It's mostly a Good Thing (TM).

But...

Lately there have been a few occasions where Henry went along with
behavior against his better judgement. Usually this has involved him
tolerating behavior at our home (we can't be EVERYWHERE although we're
always home when he has friends over) that he knows we would not allow.
He doesn't join in, just doesn't stop the others.

When we talk to him about these incidents afterward, it's clear he
knows/knew the behavior was unwise or wrong but he hasn't seemed to
have found a confident or forceful enough voice to take charge.

Any thoughts on how we can help him and/or require him to be more
assertive on these issues? Things are going so well for him socially
(that hasn't always been the case) that we are hoping we don't have to
scale back on allowing him to have kids over, although he knows that is
an option.

Thanks in advance...

-Dawn
Mom to Henry, 13