I'm just guessing here ('cause I'm clearly no vet or anything close to it), but you said he lost his appetite and was breathing heavy and drinking lots of water? I'm wondering if maybe the meds you mentioned he was prescribed could have possibly made him thirsty and sorta druggy. Also, I've read that dogs can encounter thyroid problems which can make them lose their appetite and make them feel not motivated to do anything. And I remember when I gave Tatum benadryl for her itchy skin (my vet recommended it) it ended up making her stay up all night breathing heavy.

Hope this may help.