I would just cut down a little until he is at the weight he should be. I don't generally weigh or measure my dogs food, i just go by their condition. If they look too skinny I feed more, if they start to get chubby I cut back a little. Get in the habit of checking your dogs condition by running your hand down his back once a week. You should be able to feel the ribs with little pressure. This way you notice very quickly if they have changed and can adjust their food accordingly.