My big black goldendoodle used to overheat horribly as a puppy, while sleeping. We would take ice packs and wrap them in a towel and he would sleep on top of them. Also we put a floor fan facing where he slept.

So far, we haven't had issues this year yet... But he overheats horribly if he is outside for more then 15 minutes right now during the day(all walks occur early AM or after dinner now). He likes to walk right into the shower and wait for you to turn it on.