I’d seek support from Bayswater parents in the first instance - they were invaluable support to me when I went through this with my DD.
I also would counsel to be neutral - no affirmation but no hard/fixed rejection of his ideas. Gentle exploration as to what he feels and why he thinks those feelings make him a girl rather than a sensitive and clever boy. A conversation over the fact that boys - like girls - come in different shapes, sizes and personalities. That lots of boys love ballet and art if that is in his explanation (hence the famous male people in those fields).
He is at the start of puberty. Children find this very confusing, especially when school friends will be divided between boys who have suddenly sprouted up in y8 and the boys that still look like they’re at junior school. He may also be battling with a crush and navigating early feelings of attraction and affection towards someone outside his family. Puberty is the time when children of both sexes begin to differentiate from the family unit and explore/develop their individual self (individuation). They seek connection with the community, the world outside their family unit, and this involves understanding who they are more like, who they identify with. It’s a period of investigation and exploration. An adventure even, if you frame and support your child positively.
I would find a neutral, non affirming counsellor for him to talk to (Bayswater can help here too). It took my DD 7 years to come out the other side, but we were hampered by lockdown and her neurodiversity. We have friends of my younger son, so 3 years younger, who also went through this phase but ‘grew out of it’ after a few years and are now feeling more congruent with their sex body at 16/17.
Above all, keep telling him that no matter how he feels about himself, no matter how confused he may be feeling just now, that you (and dad?) love him unconditionally and will always be there for him. No matter how he wishes to dress or present to the world, whoever he one day comes to date/love, whatever he choses to do with his life when he’s grown up… your love is unwavering and you will never abandon him. I think it is the latter that kept my DD at home, helped keep her grounded when she was deeply confused, so I’d say that repeating that message is probably the most important advice I’d give.