[Eternal]

Have you tried talking to him about this and that you'd like to know what his plans are? I wouldn't approach it from a "I need to know what you're always doing" viewpoint, but instead let him know you just want to know you're interested in what he's up to. For example, you could express how cool it is that he's going to climb Snowdon and you're glad he's doing things with friends, but also tell him you'd like to be aware of big plans like that. I don't think it's too much to ask for and I don't think it's possessive. It's pretty normal for couples to be letting each other know about plans.
As for his family, I was in a similar situation. Unfortunately, since he isn't sticking up for you it's only going to encourage his family and it tells them that it's okay. I'd also try and talk to him about this and really express how much it hurts you. I think it would be helpful if you could find out why they don't like you. In my experience, my ex boyfriends family didn't actually have anything against me, they just teased me a lot and were encouraged by my ex boyfriend not doing much about it. It may be hard, but if you feel up to it you could try and confront his family about it as well.
I'm sure you've heard all this before, but really communication is the only way to solve problems in a relationship. It sounds simple, but it can be difficult to express how you're feeling and resolve the conflict effectively. I think it's helpful to reflect back what your partner says to you to make sure you are understanding where they are coming from, and then try to find a compromise. It takes effort from both sides.