Well I had almost exactly the same treatment as you OH and at the same hospital. A scan is very unlikely. My experience is that in about three months to six months they will call him in for a review. He will almost certainly be told to get a PSA test done at the GP a week before.
I would assume the PSA will be <0.1 as he is on HT. If he has any side effects he can discuss them. They will then arrange another review every six months, assuming nothing exciting happens after five years they will say "just keep having PSA tests and if it looks suspicious get your GP to refer you back to us".
All the above is based on my personal experience, and I am now discharged from the Christie. Of course things could be different for your OH, but if you're lucky you can now put cancer on the back burner for decades.
I now cheerfully tell people, I am more likely to die on the motorbike than of cancer.