The character is a young man (around 20) and has just been diagnosed. There is no cure (or maybe there could have been had it been diagnosed earlier, but it's too late). The most he can expect to live is a few years. The symptoms are, for now, not serious enough to necessitate hospitalisation. He might suffer, but would be able to hide it from others (again, for now).
I've searched the community and googled 'terminal illness', the names of various diseases as well as visited health sites, but I still couldn't find something that would fit this situation. I'm aware it probably doesn't exist, but is there anything close enough?