I am Curious about this...

Ask him?

I'd find a way to causally ask him. What the question is, would depend on how well you know him, but if you don't know if the guy is into men or just women his own age. You probably don't know him well. Ask something about if he has been previously married or in a long term relationship.

What if you already know that he is into guys but it seems he just has trouble when it comes to being with you and he always says doesn't know why? She knows he is cheating or anything like that. They just can't figure out why he has problems now that she knows he is bi. Could he just be feeling guilty for hiding it all this time? I just couldn't help her and would love to be able to if possible.

It could b neither. Sometimes it's health and diet related others are asexual

Well, if he's bi then maybe he's more attracted to men than women? I'm bi, but I prefer my man over any woman.

If you've an honest relationship then talking about it can be key, but for some reason he may feel guilt or uncertain and not actually know the answer himself.

It could be emotional stress, guilt, or it could be diet or any number of things. Perhaps if you've both been thinking too hard or discussing this too often that could make things worse and it might be good to just relax.

Best of luck.

just ask