It's a toughie, because he could move into doubles specialism, but underneath all the mental baggage he does still have a good game for singles and I'd imagine he's still keeping up fitness work. His serve and shot speeds haven't just evaporated. I think it's all in his head or whether he wants it enough.
It's hard to say whether moving into doubles specialism would be selling his singles game short or not. Obviously his results on paper would say it's a no-brainer, but if he has one last burst of determination to fight for every point as much as he can, he's still got the shots.