My boyfriend keeps wanting to do it, but i dont know how to kindly say no

So we've done it acouple of times now and he keeps wanting to have more sex but I really don't and I keep avoiding him and going over his house but I think he's starting to catch on. So how do I kindly say no? Last time I went over his house and we didn't do anything he got all sad and I am really not in the mood for that. How do you tell your boyfriend's to chill out