Asthma might be a possibility - he has seemed to lose his voice - it just seems odd that he only coughs when he purrs. He also foams when I give him his liquid medicine for his liver situation.
He is definitely overweight - he has been on light food for a year and has only lost 5 ounces. He should be about 12 lbs - right now he is 16.5. I just read an article about putting him on canned food to lose weight - I might try it once he's better.
Thanks for the thought - I left a message for the vet and am waiting to hear back from him.