
You mshould be able to feel his ribs easily without having to dig around (other than through his coat) and there should be a discernible waist behind the easily felt last rib. If he is shaped like a tin can then he is likely somewhat overweight.
If the amount of excess weight isn't huge then I would cut his normal food ration by a quater. Always weigh his food and find a container that holds exactly this amount (you can usually find a plastic pot of sme sort that holds just the right amount. Also different foods have different shape and texture so will weigh differently so adjustments need to be made. This way I find it easy to shake a bit off good off for anyone getting a bit well covered, and rounding it to a heped measure for anyone looking lean.
Evne in the same family and height dogs need to weigh different amounts to be correct weight for their build.
One of my girls would be faat over 18kgs, but her Grandaughter who is only a fraction taller is already a nice 19kg and a just right teenager, who will body up to around another kg or so.