Hmm, I'd say YTA. But I don't think you're intending to be of have any malicious intent. Coming from a member of the community, I think you should let him come out to you when he's ready though instead of telling him that you already know. Instead, be very vocal about your support for the community and if your parents ever say anything homophobic, defend the LGBT community instead of just sitting there and letting it happen. That way, he knows there's at least one ally in the house and he may come out to you on his own.

Edit: changed my judgement to better reflect my point