Casper is a lovely boy! <3 My first cat, Kita, had the same issue, and he had the operation as well. I found that he loved the recirculating water fountains – they helped him increase his water intake and prevent relapses.
I hope Casper recovers fully. If you can, please check with another vet for a second opinion. Doing that save the life of another of my cats, all because the second vet I asked knew of an additional treatment for asthma in cats! (Bless you, Dr. Ruth!)
(((huggs))) to you and pets for Casper.