A year and a half ago, I was dating a woman before coming out to myself. I regret hurting her in the way I did, and if I could have been more honest with myself and with her, it would have been better for both of us. Sadly, I was too afraid to talk to her about it and just broke up with her without really being honest.
I think it is important to realize that coming out to yourself is scary and hard to admit. I really wanted to be in a relationship with a woman and when I found myself in one wanted it to work. I was not sleeping around on her when we were together, and it is possible for gay guys to be with a woman without kissing men, inviting people to threesomes, acting homophobic, or being vain or girly.
Being in a relationship with a women was exhausting and stressful for me. But, because I didn't want to admit to myself that I was gay, I tried hard to make it work. While I did actively participate in make-outs and romantic things, sex was hard and not enjoyable for me, but again, I participated because I wanted to prove something to myself. I think a good sign that the man you may be with might be gay is if he seems to be trying harder than he is enjoying the relationship.
Look out for close private relationships with other men. I know that these relationships were always the most important ones to me. If there are jokes about your man and his men being a couple, or if you find yourself jealous of his best friend, that might be a good clue.
If you are suspecting that he is gay, it is possibly for a good reason. I am wondering why are you asking these questions? And if you are asking these questions, what does that mean about what you are getting out of the relationship. Regardless of whether or not he is gay, you need to be getting what you need out of this relationship. Coming out taught me that.
As for what you should do, I think it is important to realize that no one can make anyone come out. Figuring out you are gay is very personal, the thought of being gay is treating for someone who has been living his or her life as a strait person. What you can do is be supportive and figure out what you need to do for yourself.
I am assuming that things have worked out one way or another in the past 3 months. But I have been in a long term relationship with a woman for that long and have slept with guys who have too. If you are still questioning his sexuality, or to anyone else who is in a similar situation, treat this guy with patience and empathy (this can be a slow and hard process) and make sure that you are getting what you need for yourself.