I'm assuming that the PSA is relatively stable, recent, and that
a recent digital rectal exam was negative - i.e., no indication of
recurrent cancer.  Since anything else is likely to be far less
dangerous than recurrent cancer, it seems to me a good idea to
check that out to be sure - though I haven't heard that pain on
ejaculation is a cancer symptom.

It could indeed be something other than the radiation.
If the burning is in the penis, there could be some kind of urinary
tract infection.  If it's in the prostate, it could be some kind of
prostatitis.  From what I've read, radiation won't cure or prevent
prostatitis and a man can have that even if he's had radiation
(surgery would normally have cured or prevented it since after
surgery there is no prostate to become inflamed.)

Some prostatitis is caused by bacterial infection and can be
treated by antibiotics.  Some is not.

I agree that seeing a urologist is desirable.  He's guaranteed
to know a heckuva lot more than I do.

Now, as a non-doctor, I'm about to give some free medical
advice that is probably not even worth what you're paying for
it, and should absolutely not be a substitute for what a urologist
might say.  But here goes anyway.

If the pain is in the prostate, he might try any of the following:

 Warm sitzbaths
 Gentle massage
 Anti-inflammatories (ibuprofen or aspirin)

I wouldn't think any of these things would do any harm.  Whether
or not they give any relief or do any long term good remains to be
seen.

If the pain is in the penis, some people recommend drinking
large quantities of water and also cranberry juice as good for
urinary tract infections - but not nearly as effective as what a
doctor can prescribe.

If any of those do work, please let us know.

   Alan