It's so hard seeing a pet in distress and hoping like hell that the vet got the diagnosis right ...

If Boo's breathing is OK and he's eating, I'd follow the vet's instructions with the pills and still watch him like a hawk. It takes a few days for antibiotics to start kicking in so you may not see improvement for a little while. Having said that, if anything changes for the worse, I'd get him back to the vet's asap. Is that awful breathing sound the same as when he was at the vet's? If it's different, give the vet a call to check if that's normal.

I know Boo is a tough, optimistic sort of guy, but he's moved half-way across the world, been shafted by the removalists, settled into a new home, become used to having his mummy at home during the day, and been introduced to a new cat - so maybe his immune system is a bit low from the combined stress. But you know him best - go with your gut feel.