OMG that photo of his poor little belly area
Honestly, at this point, I'd be getting a second opinion. I'm not a vet or a vet tech or anything like that, but I have NEVER EVER seen a little boy go through something like this after his neuter
And I don't understand why the vet feels that he needs to be hospitalized ($$$$$??) when he's just getting one injection daily
Maybe you can do another post on the general forum, asking for a vet recommendation in your area? I don't know, but if this were my little boy, that's what I'd be doing.
Sending more prayers and love to your baby boy
