I've had three shelties and never had them cut. We trimmed the butt area but not because of the heat but to make it cleaner. Shelties are a hairy breed, JMO but I hate hearing about people who cut them down in the summer. You said you have AC so keep him inside like you've been doing, keep him groomed (bath, brushing, by a groomer if you can't get the undercoat out) and plenty of fresh cold water. He'll be fine. I live in NH where it can get very hot and humid and we don't have AC except in the bedroom.