First of all, hope Mr. Big is doing well and back to his normal velcro self
I know that Toby, who is 5.5 pounds, can get very affected by the heat. He's used to being inside and we run our a/c most all the time, so when he does go out it doesn't take much to bother him. We had one episode where he was outside longer than usual, and he acted just like Mr. Big did for the next day. Sounds like you did the right thing by getting him to the vet and leaving him there to be watched. You're a good doggie mama! Good luck