Ah...I really hate to burst your bubble but after enough time on T, he will look like a man. Almost all trans men are mostly indistinguishable from cisgender men after enough time on hormone treatment. Facial hair, body hair, body shape, skin texture, scent, voice, even the clitoris, all of that changes.
Of course, you can stay if you want - it doesn't change your orientation, but he might not take too kindly to you referring to the relationship as a lesbian relationship or relating to him as female.
I can agree on is he should have told you, and he should have been honest about being on hormones from the get go. Even when I wasn't sure I was going to transition, I told my partner that I was really a man, and that there was a good chance that down the road I would do something to make my body match my brain. I asked her if she would be OK with being with a man, if we lasted, and gave her the time to really think it over before we took our relationship any further.