Here's the situation: My boyfriend and I have been dating for around 1.5 years. At first, when I met his friends I liked them a lot, but as time has gone on I've noticed a side to their relationship that makes me uncomfortable. While his friends always joked about my BF being the "dad", I have started to notice that they really do expect him to clean up after them all the time, both literally and figuratively. He does so much for them for essentially nothing in return, other than them occasionally spending time together. He puts his entire life on hold when they "need" help without so much as a thank you.
I really want to encourage him to stand up for himself, because he deserves more than this. I want his friends to at least start acknowledging what he does and stop taking advantage of him. He complains to me about it fairly often, and even when he doesn't, I can tell it bothers him. But whenever I nudge him to confront them (on the few occasions that I felt his friends really took it too far), he makes excuses for them or says it isn't a big deal. It's starting to drive me crazy how selfish they are, and how they can just get away with it. Whenever he asks for help, they won't give it or are extremely reluctant.
TL;DR: How can I encourage my boyfriend to stand up to his friends that are taking advantage of him, without causing conflict between us?