It hot weather it is not advisable to exercise your dog. He should be walked morning and evening only, when the temperature is less. Like we humans become lethargic in hot weather so do dogs. Keep him in a cool place inside the house and give a lot of fresh water in a bowl to him to drink whenever he wants. He may also eat lessto the high temperatures these days.
Generally labs are people dogs and hence they are popular as family pets. Guarding is not in their blood, but if someone threatens their owners I am sure he will come to your rescue. Generally younger pups can be trained to bark on command, but in your case your dog seems to old to be trained for that. Just enjoy your dog and take good care of him and do basic obedience training.