About ready to give up trying

So our youngest kitty, he is two, had a bad UTI and we had to take him to the emergency clinic at 11:30pm at night, came home at 1:30am, then two days later back to our vet’s office because he could not urinate and when the vet squeezed his bladder he passed a long blood clot.  They gave him a shot of antibiotics that stays in his system for two weeks, we had to keep him in a kennel cage for about a week, and then get a urine sample to bring in.  The vet also wants him on a diet, as he weighs 20.80lbs.  He is just a big boy.  He refuses to eat the science diet food and the special care formula food didn’t agree with him either, if only there was weight loss pills for kitties!  Anyhow I have not been able to catch a urine yet.  I have tried putting him in a kennel cage with an empty liter box, that didn’t work, trying to watch when he heads off to the main liter box with a dixie cup at hand, its just impossible for me.  With hubby on crazy hours, as soon as they settle we are just going to have to make an appointment to take him in and let them catch it and pay for the visit because I have tried everything I know.  He is much better and back to himself.  As far as his weight, he isn’t overly chubby, he just is a big cat!  Has big paws too!