Hi Sarah.
Welcome to the forum. I'm sorry that you've had to join us but by doing so, it's a sign that you care so much about your dad.
As Chris says, it's common to find a biochemical recurrence after a prostatectomy. Your dad will probably be given follow up treatment which may include radiotherapy and hormone treatment. It's called salvage treatment, it's a term I dislike as it reminds me of salvage yards, full of clapped out cars.🙂
I wish you, your dad and the rest of your family well. There are loads of kind, helpful, supportive and knowledgeable folk on here who will be able to assist you all on this phase of your journey. Good luck.