You are pushing it too hard, let him adjust gradually. My friend had similar issues, and from his experience i suggest you
try the wheelchair three times a week, an hours each time. That's for the 1-2 weeks. In the 3-4 week increase the frequency
to 2 hours, and so on. Hope this helps.

I had experienced similar issue a few years ago. My can was left in the cold by mistake, after we went for a vacation of five days. I think it had the same symptoms you mentioned (peeing everywhere). I think he got into some kind of trauma from what happened. Anyway, in our case, we took him to the ...