It is not uncommon for some men to have a low libido, and it has nothing to do with the woman they are with most of the time. Low T could definitely be it, as well as other medical or psychological issues such as stress. If he is going to the doctor to try to get to the bottom of his lack of desire I would wait to see if you guys can get back on track in the sack, but I would not jump into marriage just yet. A year is not that long and you want to be sure that you will be compatible in all ways before saying I do. I know that for myself no sex in a marriage would be a deal breaker unless we agreed to a more "open" arrangement.
Added: While I agree with some of the other posters that there is of course much more to a relationship then sex, that does not make it unimportant. Some people can be very dismissive about issues like this, or try to make you feel badly for considering sex an important part of your life, don't let them! Often that is simply because sex is not important TO THEM! Don't let them make you feel guilty for knowing what you need to feel satisfied in a long term relationship or for considering something that is normal and healthy important. Sex increases closeness and intimacy and for many it can be very difficult, if not impossible, to feel truly connected to a partner if you are never, or very rarely, together in that way.