I think I'm with @stage4wife here. Just be honest and tell him that.

When I went through treatment, my wife and I had been married for 30 years. If we didn't have that long loving relationship, (she is absolutely amazing in many ways), I don't know how we would have gotten through it together. It's difficult and when you are diagnosed, then deal with the after effects, it's absolutely a punch in the gut. It has to change one, at least temporarily. My wife gave me what I like to call "caring space", she let me alone most of the time but patiently listened to me complain when I had to. I would hate to think of going through this completely alone, hopefully your friend has some family around.

If he had radiation, he's probably also on a drug which essentially destroy's his body's ability to produce testosterone, he likely has no sex drive at all, or very little. The taking of that drug will be temporary, but, depending on his medical team likely somewhere between 6 mos to 2 years. I would think that at his age, once the drugs are out of his system, some, if not all of that drive may return.

If he was diagnosed in Oct, he was likely not completely finished the radiation until sometime in Dec at least. Now at the end of January he's likely just recovering from the side effects of the radiation itself, mine weren't horrible but it effects people differently. Since this is his primary treatment, his dosage of radiation was likely higher than mine was. He could have had some irritable bowel type of issues. In some people, it can also cause almost debilitating fatigue. My radiation oncologist said normally it can take 6-8 weeks for the side effects to subside significantly. My point being, he's still pretty early in the recovery from radiation phase.

I would suggest giving him time as well to heal both mentally and physically. Perhaps, suggest a get together where he wouldn't feel any pressure sexually, Meet for coffee when he would know you have limited time or something like that. Something less "date like" than you are used to. Maybe a breakfast?

It sound to me like he may be "shooting himself in the foot" as we used to say by staying away from you. Obviously have to move on with your life at some point for your own mental health, but, generously texting him a good word once in a while may be good therapy for him as well.

Best of luck to both you and him!