Welcome to the forum! Together, we will do everything possible to help your puffer.
Loss of appetite can be caused by a few different things, such as poor water quality, internal parasites (IPs), or some other stress such as aggressive tankmates. Is that a pic of your puffer you're using as your avatar? If so, he doesn't look all that skinny to me, just like he needs a week or two of good meals. Have you seen his poo? If so, what does it look like? If it looks white and stringy, that's an indicator of IPs and you would want to start treatment immediately.
You mentioned that you fed him some week-old shrimp. Was it cooked, or raw? I would probably avoid doing anything like that in the future, as seafood spoils rapidly even when refrigerated. As for feeding in general, look here for some good reading:
library/category/feeding/. The gist of it is that they need shellfish, crustaceans, and snails, supplemented with blood worms, mysis shrimp, stuff like that. Nothing cooked, only raw. You might try enticing him with some frozen bloodworms or mysis shrimp, just to get him started. Or, go to your LFS and see if they have any pest pond snails infesting a tank - maybe they'll give you some.
You also mentioned that you were nervous about snail breeding. Well, with a puffer in the tank, that particular problem is gone, period. You'll never have to worry about that again. So go ahead and start feeding him snails; most puffers find them irresistible.
When was the last time you did a water change? I ask because as I said above his behavior could be caused by poor water quality. If it's been more than a month ago, I'd do a moderately large water change, perhaps ten gallons or so from your 28 gallon, and see if that helps him. Water quality is the single most important aspect of keeping a puffer healthy and happy.
Fast-moving, aggressive tankmates can definitely stress a GSP. I tried keeping mollies with one...they bullied the puffer to no end, and he wouldn't come out of his hiding hole to eat until I got rid of them. So that's another consideration.