Hi BBE,
If your Dad has been on HT the chances are he had Radio or Brachytherapy. It's quite usual for the PSA to rise post treatment, anything up to two years is not unknown, but it's more to do with the good cells in the Prostate regenerating having been zapped along with the cancerous ones during treatment. As long as it stabilises there's no cause for concern.
A lot of good information is here:
Prostate Cancer UK