If a man (96) has prostrate cancer what are the signs the cancer is growing?

More likely to be constipation, I'd have thought, though possibly still because there's less space than there used to be? But n.b. this is purely amateur comment; plus, far more importantly, the poor man is in discomfort and it should be reasonably straightforward to relieve that discomfort whatever its cause. So get him to a doctor for professional advice a.s.a.p.

It's prostate, by the way. And rectum. Just so you know, and so junior doctors don't titter behind your back.