I agree with others - it’s not selfish of you to not want to spend every weekend at his house because he chose to get a dog or something he can’t easily bring over to your house. The reasons he might want a pet are understandable, but if he’s serious about getting one he needs to consider the animal’s welfare first and foremost, how does it really fit in with his life, how would he juggle the pet and your relationship in a way that was fair to both (or all) of you, who would look after it at short notice/emergencies if needed etc (not just going on holidays/travelling). Etc.