If the difficulty is getting an erection, there are implants that can be used. They inflate the penis through a valve/pump inside the testicles.
http://www.seekwellness.com/mensexuality/penimpl.htm Of course now days there are all of those new medications which have removed the need for this implant in most cases. There are still a few cases out there that need the mechanical prosthesis.
There may be underlying psychological issues that lead to limpness – I do not know if your husband has had therapy for dealing with the implications of having prostate cancer and then its removal. These things can have huge impacts on males and their performance issues – even if they are not consciously thinking about it.
As for the burning, I can not find anything beyond STD's and the “cures” for that.
It is possible that since intercourse is rare that his body has yet to adjust to the dry ejaculation – men, even healthy men who do not ejaculate for a long period of time (nor have wet dreams) will have burning sensations, minor pain (testicular pain) and a few other issues when they do climax. (Side note: for any other readers any burning or pain should be taken to a doctor since they can be symptoms of other issues)
There are minor, chronic infections which can be missed which can lead to similar sensations, the urologist may have tested – or may not have considered infection however everything from bladder to testicles could be host to an infection that is chronic and unnoticeable until ejaculation time.
Some medications and even some over the counter medications and “herbal” or vitamin supplements could also lead to a similar issue of burning.
Diet can play a factor (It also can cause the prostate cancer as well). Exercise or lack there of can also play a factor.
There is a wide variety of combinations of all of the above (and I assume other things as well) which can be playing their roll in the burning sensation.
I would make certain that we have ruled out infection first, then rule out medications either through ending dosing (in herbal and vitamin and OTC non-prescribed medications) or switching to similar medications for other health issues – This should be done under a doctor’s care.
If that failed to work I would examine the diet (Add here food allergies as well). It could be just one, or a combination.
I would go over all of these possibilities with the Urologist and keep a check list of “things we have tried” for yourself.