It reads like your lil' furball has E.P.I. - exocrine pancreatic insufficiency.
It is necessary to monitor your cat's progress on a weekly basis after initiating treatment.
If he is wasting away and your vet has no further treatment options you can always get another veterinarian to diagnose your cat, but, if you haven't already, you might also want to have "that" conversation with your vet. Do you have a good report with your vet? Do you feel like they listen to your questions and take the time to make sure you understand what's going on with your cat and their care? If you don't trust your vet, find another one.
Beyond palliative care and consideration of your pets comfort, just give him all the love and affection you can while you are both blessed enough to be with each other. The passing of a beloved pet is never easy. Take the time now just to be present in the moment with him when you can. Your vet should be able to advise you of what to look for if his condition worsens, or what to consider for making an end-of-life decision (mobility, ability to do his business, alertness, pain, lethargy). Has your vet suggested any particular dietary guidelines? From the above article:
Avoid high-fat and high-fiber diets, which are more difficult for digestion.
...but in the final days, maybe just spoiling him with treats and catnip and such is alright?
You can always get another vets prognosis, but at some point, if you haven't already, do make the time to think through the logistics and make a plan for euthanizing your dear friend. Your vet should be able to advise you about the particulars. This is also a conversation you can have with more than one vet. Also, do consider what your options are if your cat passes naturally. Planning for the inevitable will help ease the emotional burden of losing your buddy when it happens. As much as I wish I had a less morbid answer, you'll always know you gave them all the love you could while they were here.
Little furballs... They sure have the worst habit of not living forever.