I don't think you're being rude inviting yourself along with his brother/friends. I assume his brother knows about you and won't "tell" on your boyfriend? Does the brother have a girlfriend he could bring along so you could sort of get to know friends of his friends :-)

How far apart do you live? How did you meet? I don't think I understand how the relationship began and is maintained for so long if you don't see one another much and don't inter-relate with each other's real lives?

But I'm a little concern with how he is "allowed" to go to the beach with his brother and "fool around" but not go out of the house normally? I have three brothers and they were out "driving" in the car whenever they could; the only time I ever ran out of gas was because my next-older brother ran it out of gas just driving around. That's what guys do; I can't imagine what his mother wants from him inbetween graduating and getting into med school? How does going out relate to that?