If you're not sure if he's your boyfriend, then he's not. There's no such thing as a 'boyfriend by default due to lack of others'.
However, you've been dating him for 4 months...so you should at least talk to him about this. Do you want him to be your boyfriend? Not necessarily thinking further ahead...but do you want to be in a relationship with him, however seriously you might want to take it? When you know that for yourself, talk to him. He has a right to know where he stands...just 'a' date, boyfriend, serious long-term partner...
Good luck
Morgawse Sedai
Mornings happen to other people.
Passionate Heerenveen Fan
I survived Michigan 2004