I would call the vet to see if you can get him in sooner if possible. During the appointment I would make a point to ask about the condition of his heart ie does he have a murmur or any abnormal sounds etc, as well as the condition of his lungs
Are you in a heartworm prevalent area?
What are you feeding Max? I'm not familiar with diabetes but we do have a member who is, I'll pm her the thread perhaps she'll have some suggestions for better regulating the diabetes.
When did the allergies start and has the vet determined the cause? Seems like an awful lot of meds
In the meantime since it has been so hot here and will continue to be hot all week - skip the walk.
let us know how Max is doing