What to do about my overweight dog?

I have a male 4 year old Blue heeler/pit mix. He is way over weight. we give him a cup of food three times a day. We live on 4 acres and gets free range when he is let out plus he goes out and does chores with me (goes down a hill/back up to the barn and out to the horse field at the back of the property) and when I go for walks he goes with me. He isn't ever locked in a cage. He doesn't really get scraps, doesn't have worms. Any suggestions?