did the Vet do blood sugar tests? or Thyroid? hows his appetite? is he still eating good but losing the weight?
to loose that much weight so suddenly something is very wrong....
To get a stool sample your just going to have to keep him indoors with a litter box until he cooperates...
sodium free chicken broth is okay to add a small amount on a regular basis if it encourages him to drink more....but I wouldn't do the tuna, cats really aren't supposed to have any of the canned tuna, too high in mercury and that would also be present in the juice.
is he on canned foods? thats where he needs to get the moisture from,dry foods only contain 12% moisture, cans contain 78%.....alot of difference and most cats do not consume nearly enough water to compensate for the dehydrated dry food...
Ollie is a male and therefore real prone to FLUTD if he is on dry food....I encourage you to read a post and reply I gave to bkitty tonight....Ollie will get this same issue if he is on dry food and his urine is concentrate already!!!!
so please read...
http://www.medhelp.org/posts/Cats/My-Milo/show/1980314