Straight Friend Dating Girl, Could be Gay..

I have a friend, let's call him Jeff, who I have a feeling may be in the closet. We are both in high school, I am out, and we are very good friends and hang out a lot with a few others in our friend group.So, before I became friends with him, people very frequently speculated that he could be gay. He fits a lot of stereotypes, all of his friends are/have been girls or LGBT guys. His mannerisms are fairly effeminate, and often times people within minutes of meeting him assume he is gay. He gets touchy with me in subtle ways, hugging me and holding it for a little too long (to the point where other people have mentioned it to me), and just generally talking/touching me in "jokingly" ways that most high school guys would not joke with their gay friends. Similarly, sometimes he is over-the-top disgusted talking about homosexuality, yet other times he doesn't care as much and will even ask me questions about it, which makes me think he is working really hard to convince people he's not gay. Often times when he talks about girls it feels off and not natural. However, it does get a little more complicated.This year, he got involved with another one of my friends, let's call her Sandra. Toward the beginning of their relationship, she was very concerned with him being gay and talked to me about it. Now, their relationship has picked up and they frequently have sex. Knowing this, people still think he's gay. talking with him about it is sometimes weird. I remember one time in particular he was acting really defensive for no reason, making sure to tell me he initiates the sex most of the time, as if he needed to convince me.The problem for me is that I want him to be comfortable being who he is if that is gay, and most signs point to yes. It's hard to articulate how overwhelmingly literally every person with the exception of his girlfriend think he is gay, and even she has had concerns. I hate to be one to act on stereotypes but I feel with knowing him very well personally that he is most likely gay. I see him react to things in the way closet-me might have. I feel like this is harder than just letting him know I support him & he can talk to me, because it would hurt Sandra to even suggest my thoughts or for him to suggest he might not be 100% straight, and worse, she would be incredibly hurt if he ever came out. This whole situation stresses me out because I want him to know he can confide in me, but if he is in the closet, he is verryyyyy deep in there, even though everyone assumes he is gay.TL:DR, friend everyone assumes is gay dating another friend (female), and I don't know how to offer support