While it's impossible to know for sure, there are some signs that your boyfriend may be gay. These include frequent pressure for a threesome with men and a lack of attraction to women. He also might frequent gay-friendly spaces and have a lot of men friends on social media.
If he doesn't talk about his previous relationships with women or is hesitant to discuss his own romantic history, this could be a sign that he isn't comfortable talking about sexual orientation. He might be afraid to come out as gay for fear of the judgment of his family, town, or society. This is understandable given the amount of hatred and prejudice towards gay people.
Another big red flag is if he is more into non-verbal communication with his male friends. This can be as subtle as a lingering gaze or man-to-man hug. He also might be more into playing with toys that imply a desire for man-to-man sex. This could be something he does for fun as a way to explore his sexuality or it may be a clue that he is gay.
He might also talk about his own struggles with being a gay person and how he isn't sure if he is. While this is a painful part of being gay, it's a good indication that he might be gay. He might not realize it himself and be avoiding the topic to avoid being hurt. It might even be a subconscious way to cope with the fact that he's gay and that isn't something that he's comfortable owning.