In my opinion, crossdressers are born, not made. I started at 4. So possibly he’s had it in him all along and he’s just now comfortable enough to mention it, maybe ask him if that’s the case. It’s possible, I guess, that he’s just thinking about it to make being bi “easier”, not sure though.
Id love to share my crossdressing with my wife and have her be supportive and encouraging but I highly doubt that’ll ever happen. The ones of us who do have that are very lucky and should be very thankful for such a spouse. Regardless, if he’s truly a crosssresser, even though you aren’t into it, maybe you could allow him some private time to experiment with it and enjoy himself.
Or if you’re the type to at least try once, maybe let him dress up or even help him and take it for a spin one night. You might like it more than you think or you might realize you dislike it a lot more than you already feel you do. In that case, try to be accepting enough to let him do it alone. I can tell you from experience, if it’s in him, he’s gonna do it regardless. And it really sucks to keep it from your spouse and do it behind her back, I feel so guilty for doing that but it’s my only outlet.