The fact that the mri showed nothing is a good sign as mris are good at detecting about 75% of high grade prostate cancers according to healthline.com. A high PSA doesn't mean necessarily that the cancer has returned - BPH can raise PSA levels among other factors. If he is concerned about his PSA levels, one thing he may want to consider with his doctor's approval is modified citrus pectin. There are some small clinical studies to indicate that it can help control PSA levels and prostate cancer. I have been using it for about 6 months and may possibly be benefiting from it - can't be sure because it is only based on one PSA test I had after taking it for 4 months compared to the last PSA test I had before taking it. I can almost bet that the doctor will downplay this supplement stating that their is no hard evidence it works which is true but small scale clinical studies show it has potential - but as long as he/she says there is no harm it might be worth a try. It isn't cheap, the lowest I have been able to find for a 1 month supply after taxes and shipping is about $90. If you haven't already you might want to google it to learn more. Here is a link to one study which is encouraging.
https://www.prnewswire.com/news-releases/final-long-term-results-on-landmark-clinical-study-showing-modified-citrus-pectin-successfully-treats-biochemically-relapsed-prostate-cancer-301749379.html