Hi, Girlfriend. I am a bipolar who has been in romantic relationships with bipolar women. First, the same general rules of relationships that apply to everyone else also apply to bipolars. We're not exempt from the rules, nor do we deal with rules no one else has to deal with. Having said that, having bipolar undoubtedly adds some complications. Any transitions or break-ups are fraught with emotions on both sides, and - you may have noticed - another name for bipolar could well be "emotions amplified."
Bipolars are especially vulnerable to stress, and relationship issues come bundled in stress. There is no easy way to negotiate a break-up or transition, but it will help if you can do everything possible to make your boyfriend feel emotionally safe and secure. No doubt, he will want to know your reasons for "rejecting" him, and it pays to be honest - but you need to figure out a way to do it without him interpreting your explanations as a personal attack or as a disparagement on his character.
I think it's natural for any rejected partner to want to turn "platonic" back into "sexual." Or, if that is not possible, for the rejected partner to view options as all-or-nothing. Too oversimplify, the person initiating the end game is invariably the one who sees reality while the "rejected" partner is generally living in a dreamworld.
It takes time to work one's way to acceptance of reality. One of the phases after "denial" is "bargaining." My guess is your boyfriend is in that state, hoping events will resolve back to the good old days. And, if he is feeling strong emotions - bipolar-type emotions - it may take a little bit longer to work through this.
This is where depression can be good. Bipolar comes wrapped in depression, and you can look at depression as a state where the rose-colored glasses come off and reality sets in. I would never wish depression on anyone, but if you are picking up depressed vibes from him this may be a sign that he is working his way to acceptance.
Whatever happens, if you are picking up depressed vibes, don't allow yourself to be manipulated. Trying to make him feel better may be counter-productive. The sooner he accepts reality, the sooner his psychic being becomes free, the sooner he heals, the sooner he is ready to move on.
Another point: Helen Fisher and others have done research noting that break-ups tend to re-ignite all the "passion" hormones. Your boyfriend will most likely experience this, and you may, too. It's an added complication that you need to be aware of.
I know this is a rambling response, but hopefully you can extract a few sensible suggestions from it.