Yeah, you'll have to cut all food but kibble. Iams isn't a great kibble and may be contributing to him being overweight. Try switching to Taste of the Wild, or Royal Canin.
Royal Canin has a Lab forumla. You should feed 5 cups of kibble a day. Split it into two meals of 2.5 cups.
Taste of the Wild should be 4 cups a day split into 2 meals of 2 cups.
He will need a lot of exercise, but it will need to be low impact.
Hw should go for two 30 min walks twice a day. (stroll if you can to keep the impact on his joints low)
If possible, take him swimming. Labs were bred to swim and it'S great, low impact exercise.