Just looking at the photo its hard to tell, since the poop itself looks quite big for such a small puffer...
My question regarding his poop would be;
1. Was it stringy looking when it was finally all out?
2. Did it look like a loose strand of pearls? Something a but like this 。---。----。----。
3. Did it look like a small lump / like an elongated pellet???
A cherry red shrimp setup would work, but you would need to start off with a fairly lrge colony to keep a puffer fed, even if only sparingly...
If he attacks snails, when you do get them, maybe feed him using a pair of tweezers or something so you can make sure hes eating...