The weight will differ from dog to dog as they will too differ in structure and size based on their parents etc, even though they are all labs.
The best way I use on my dogs is if you can feel the ribs and count them on your dog then he or she is having a good weight. This does not mean that you can see the ribs coming out when you look at the dog, but can only feel them when you feel your dog. If you are unable to feel the ribs and only fat is what you feel then the dog is fat.
labs love food so try and keep a check on that as they will always make those eyes to get the food or tit bit that you may have and then it gets very difficult to make them loose it. Also young dogs, specially should be kept lean for better growth and to avoid health problems later. This does not mean that you starve your dog, but feed him a wholesome meal. Royal canin has taken out a food just for labs based on their age, it should be good. Also feed your dog some calcium and vitamins based on your vets advise. All The best and enjoy this lovely dog.
PS: Due to the heat please keep your dog in a cool place inside the house and walk him only in the morning and after sun down. I am hearing a lot of cases of supposedly heat stroke. Also if you exercise him, cool him gradually and do not get him into a ac room etc immediately after exercise.