Generally a number that high would be considered a strong indicator of PCa.  However, your dad's prostate size is also on the extreme size and the two are directly linked.  So he might have PCa, or might not.  

Fortunately, cancers found in guys that age are usually pretty slow growing and not aggressive, so the two month wait probably is not something to worry about. Regardless of the new PSA, I would be asking for a biopsy just to be sure.

Biopsy is the best way to tell.  I suspect that the urologist may want to take extra samples (more than the usual 12) with a prostate that big; make sure that your dad is given some sort of pain blocker for the test.  Some doctors still do the biopsy without the pain meds.  It happened to me and I passed out from the pain.  Although not everyone finds it that painful, there is no reason that it should happen to anyone.

At 75 he is at an age where "watchful waiting" is often suggested.  It is my opinion that linking that treatment to age is an out-of-date concept.  We are seeing much greater life expectancies these days, with men working and being active into their 80s and more.  Making a decision to treat vs. watchful waiting means being very honest in evaluating his current health and health prospects.  Certainly if he has an active sex life (something that will be difficult for him to speak to a daughter about) he will want to give watchful waiting a higher ranking.