You can only have one BF at a time and here's how you get a minor BF:
To get a Clique Boyfriend you’ll need to get his Affection to max and meet a specific requirement. The requirement for each guy is-
Jason; Get the other nine members of his Clique to 100 Affection at least
Ralph; Unlock 5 date locations in one playthrough
Rex; Get all 7 Side Conversations with someone other than Adrian and Kam
Mateo; Have him at Max Affection, get to Level 3 in any part-time job
Davis; Have Nerds at 1,000 Reputation at least
Waldo; Have at least 200 Reputation Points with all six Cliques
If you’ve met both of those steps a pop up window will appear asking if you want to make the guy your boyfriend.