Frankly, he doesn't get to say no sex AND no therapy and not expect you to leave. He's taking away an important element of a relationship and closing down any discussion on the subject; this is not a healthy way to treat an adult you love and see as your equal. .....And you can tell him that's from a therapist.
The thing is that there are still lots of reasonable questions. Penetrative sex is only one part of sexual activity, which bits can he do and which bits can he not? Age/tiredness would not prevent ALL sexual activity (touching, kissing etc.) Why is he okay with the drastic change in his sex drive? Sex is usually a good thing that most people want in a relationship at some level, so his lack of sex drive and instant acceptance of it, seems odd. Which brings us onto the difficult bit. Is he getting his needs met elsewhere with an affair? Is he gay/bisexual and struggling to find you a turn on? Or is there some physical change to himself or you that is affecting his sex drive?
Ultimately, a sudden lack of sex drive does happen, that is not the worrying bit. The bit that is really off about this is his chilled acceptance and refusal to talk about it; like he knows exactly what's going on but doesn't want to share. If you've not already, obviously ask directly, what's going on, are you having an affair? Etc etc. If it is about your appearance then it's difficult but at least you have something to work with. Telling you nothing, is cruel and disrespectful..
You can go to relationship counselling on your own if he refuses, it's quite a common occurrence. It will allow you to work out where you go from here. It might encourage your partner to come along to a session if he knows is your counselling and he's not obligated. This really won't go away on it own action needs to be taken. It's not just about sex, you need to feel desired in a relationship regardless of what level of sexual activity you engage in. It doesn't feel like he's giving you even that. Whatever you do make sure you're talking to him about it and keeping lines of communication open, even if he doesn't reciprocate or acts like he doesn't want to hear it. Once you retreat to your respective corners and stop talking to each other about personal stuff, then the relationship really is over.
I hope you work this out x