As regards scans, the best one that I know of that is available in the UK at present is the 68 Gallium PSMA which can show spread to bones and lymph nodes. However, the chances of it showing cancer cells with such a low as your Dad's PSA are extremely slim. Furthermore, this scan does not work for between 5 an 10% of men. There are at least 4 centres in London that can do it and prices were circa £2600 when I last checked. (The Royal Marsden gave a rather higher figure.) Some of these facilities offer the scan on the NHS but I think it very unlikely that the scan would be offered within the NHS with such a low PSA.
There is a better scan on trial in the USA, Canada and a few other places but not yet in the UK to the best of my knowledge. It's called 18F DCFPyl PET/CT but this too is unlikely to show cancer cells with such a low PSA. There are a few other scans and variations on present ones being developed. If indeed there are only two centres doing a scan that Dad talks of, we would be interested to know the name of the scan and where it is available, as all this sort of information adds to our collective knowledge and can prove useful.
Meanwhile, it's really a matter of monitoring PSA (3 monthly intervals seems good to me) to establish rate of rise if/until it reaches the 1. at which point his Oncologist wants to see him again.