OK - I think probably the most important part of the vet's warning was probably against the ~work~ and huffing and puffing that would make him hot and sweaty.
If he wants to stand in the sun when he has shade available, I am not sure I'd put him in just to get shade.... For his lungs I'd worry more about the dust in those hot corners they like to stand in. Maybe hose that area down, or maybe, with all your rain dust isn't a problem.
Dust is always a problem here. That's why I don't stall. I can wet them down good, and in a short time they're dusty again. The only thing that really seems to get them in their stalls is flies. If the flies are bad they will go in.
Well, you all seem to agree with what I thought. Give him his own choice. I've just wondered about it. Thought I'd ask. (Anything to get out of housework:), I'm almost done in the kitchen tho.
Becky