Why is the guy i have a thing with being like this?

okay so me and this guy have had a thing for about three weeks now to short of a time before dating so im just waiting until he is ready i guess... and he is 17 and im 15 and we have done stuff together does it mean he is using me im not easy and i dont like to put out a ton but i still think he might be using me i really dont know? and like everything i do lately is bad or wrong and once again he also talks to my best friend who is in 8th grade he flirts with other girls and talks to other girls and idk he tells his friends he likes me alot but he doesnt act like it and im just really confused on what to do and like he calls me fat but that cause i call myself fat and like he calls me a hippo as a joke but it gets annoying and idk anymore on what i should do please help!!!!!