This is not as easy a decision as you would like because of the pros, cons and risk factors. There is no one choice fits all as you already know.
You didn't mention if this elderly gentleman was also on medication for the BPH. There are many pros and cons. I assume that at this point he has tried both meds Flomax (to help flow) and Avodart (with a 50% chance of reducing BPH) or they have not been considered because of his diabetes. Some PwPs have issues with the side effects of Flomax, others like my husband (how has BPH and PD) do not. We will be discussing Avodart at his next appointment with his primary care doctor.
Before moving on to the stent, is it possible to include diagrammed instructions for care of the supra-pubic catheter in a way that nursing staff cannot miss them. It would really be a responsibility of his doctors to make sure that the instructions were both charted and physically demonstrated to all personnel who care for him. It is more than mismanagement.
Since he is living at home, is there a caregiver/day nurse who comes to the home to assist with bathing, cleaning the cath and such?
http://www.drugs.com/cg/how-to-care-for-your-suprapubic-catheter.html
Is he using a night bag properly hung? Is he getting enough fluid? This is very important with the supra pubic catheter and many men with BPH don't drink enough out of years of habit.
There are other risk factors associated with the supra pubic catheter and these may outweigh the advantages in this case.
http://healthguide.howstuffworks.com/urinary-catheters-dictionary.htm
http://www.atlasofpelvicsurgery.com/3BladderandUreter/1InsertionofSuprapubicCatheter/chap3sec1.html
There are the usual surgery risks for a diabetic as well as the possible need to repeat the urethral stent surgery at some future time - which makes this a difficult call.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1472823/
You've got a lot of reading to do because there is no one right answer. However, since there are already some serious issues with the supra pubic catheters, it makes sense to consider a stent to reduce long term risks. The next stent is probably the least invasive an offers placement in the bladder or the prostate. Because of both Parkinson's disease issues as well as BPH, it would require some serious discussions with his urologist.
http://www.urolume.com/consumers/urolume/
Note that there is an 866 number at the bottom of the page. I suggest calling it to learn about aftercare while considering the direction of possible change.
The next stent is the kidney bypass:
http://www.surgeryencyclopedia.com/St-Wr/Ureteral-Stenting.html
I also suggest reading the abundant information about the kidney to bladder stent at the UK site below:
http://www.bui.ac.uk/PatientInfo/ureterstent.html
Sounds to me as if you have a spread sheet ahead of you. The patient needs, the risk factors, the options available, short term, ongoing care, long term goals and needs. The more you can compare based upon this individual, the better prepared everyone will be. Of course the long term goal is maintaining the best health possible for this man.