Blossom,
‘They’ do say that in his situation a Gallium 68 PET-PSMA scan might be appropriate (that’s if you can get one on the NHS, otherwise £2600 in UK, £400 in India), as that may pinpoint the tumour(s) and enable further targeted radiotherapy to attempt to kill off the cancer once and for all.
As Chris says, this is only successful if the cancer is large enough to be spotted on the scan.
Best of luck.
Cheers, John.