This is a challenging scenario. On the one hand I can see why you’d feel a bit disorientated because all the perceptions you had regarding sex between the two of you have been altered by this revelation. You may also be wondering if there is an ulterior motive other than just being honest - does he now want to physically experiment?
But - it could just be a confession that he finally feels comfortable enough to make to you and is (finally?) brave enough to admit. He is putting himself in an extremely vulnerable position and obviously trusts you a great deal.
He may have questioned his sexuality for decades or it may have evolved over time. But the point is, just bc you have found out he is attracted to men as well as women, doesn’t mean he doesn’t absolutely love you, or suddenly finds you unattractive and wants to leave.
Most ppl are attracted to other ppl while they are in relationships, it doesn’t mean they would act on this though and the vast majority don’t. Gay and heterosexual ppl are capable of crushes and finding other ppl attractive but can and do remain faithful - being bi-sexual doesn’t make you any different. Also, as individuals we aren’t programmed to automatically be attracted to absolutely every person matching our gendered sexual preferences when we encounter them. It would be exhausting!
I can see how you may be questioning a lot of aspects in your relationship - past and present - but unless he has said he regrets never being with another man and it is something he would like to do, I’d cross that bridge when you get to it. It’s different I know, and I don’t mean to trivialise it, but it’s a bit like finding out your partner also actually likes a certain type of person - blonde/tall/academic/norwegian - it doesn’t mean he’s not totally in love with you and doesn’t want to stay with you, he’s just attracted to other types as well. As we’re all allowed to be.
Put another way, he’s not saying he’s gay, is not sexually attracted to you and can’t be with you any more, he’s just opening up about a different facet of his sexuality to be more honest with you and let you understand him more as an individual is my guess.
It sounds like you are sympathetic to his situation and being very understanding and supportive OP, which is really positive and I’m sure he truly appreciates that.
Unless he gives you something to feel insecure about - and you’re entitled to ask whether he wishes to pursue something with a man as he hasn’t in the past - just do what you’re already doing and try not to feel insecure, difficult I know when something like this happens presumably out of the blue.
I think though, if he does want to explore that aspect of his sexuality it would be understandable if you no longer wanted to be in a relationship with him, if you value monogamy. Announcing that you are attracted to other ppl doesn’t give you a green card to pursue sex elsewhere.
He may feel it’s something he needs to do, he may want to stay with you, but just for you to have a fuller picture of him. Is it something he has just discovered about himself? Has he harboured these feelings for decades? Only discussing it with him will make things clearer and he owes it to you to be honest with what he wants in your relationship and how he sees your future.
It sounds like you care about him a lot and are extremely supportive OP, he’s lucky to have you.