Struggling with steep hills

[Cobwanted]

My 15 year old cob seems to be struggling when walking up quite steep hills. I hacked him this morning after a few days off, and he was really struggling. The hills in question are quite steep - I get puffed walking them with the dog. He was really puffing and blowing, and I could feel his heart beating!! I thought he was going to have a heart attack!!! He is kept on grass, with no hard feed or supplements. He is used as a happy hacker/ family horse. Do you think he is just unfit? Or could there be something more serious??

 

[ThePony]

How is his temp, is his breathing rate normal? Does he have any thing running out of his nose or has he coughed? What is his weight like? Does he usually manage these hills well?