If people don't want to feel deceived does my partner now have to announce to everyone he's trans?
As someone who was socalised as a female the usual social norms apply. Females imo would have the advantage in they will be taught safeguarding from a female perpective so have a more subtle understanding of why something 'small' is important.
I dont need to know the sex of the person in most social context but sometimes I do.
Oddly enough because the physical contact barrier/taboo is based on the male standard prototype doing X&Y your partner would be asking "what should a man not do that yet woman can", so I expect that a lot of social taboo are beneficial for F to M.
If you refer to him as a man is he obligated to correct you?
Why would it matter if my observation led me to an incorrect conclusion (or I am polite)?
Unless the situation led me to do something I would not have done/agreed to had I been aware your partner was female. That is breaking the social contract your partner knows exists.
He uses the men's washrooms because he looks like a large hairy buff man.
That is making a choice and taking on the risks attached. One of which is the men objecting
Should he be entering women's washrooms even though he looks like a man?
Again this the social contract around making the choices and risk transfer. Your partner knew that the visual changes would create an issue and the social contract (for women anyway) is to not transfer that out on to other people.
So can but should not so a third alternative is meeded if men are not willing to share.
Would he still have access to medical care as a trans person who requires testosterone?
Your partner has no medical need to access a drug used in a medical condition context.
That is the direct result of TRA activism and in the US all drugs and proceedures are seen as lifestyle medicine so not covered by public funding and are sold as an addon product (similar to birth control and pregnancy cover)
So its a mental health issue managed by drugs.
Doctors manage it on the same basis that say depression is managed. However as there is no firm science or pharma is off lable doctors should be justifying treatment by regular assessment (looking at the whole of your partners health phyaical and mental)
Or would he be forced to detransition?
Its a question around should the drug be deregulated for a specific group for a specific need. Provided it remains recognised as a mental condition that problem should not arise.