The ideal way to find out if he is overweight is to have your vet or a tech evaluate his body condition. There is a scale that we use that ranges from 1 (very thin) through 5 (obese) with 3 being the idea body weight. Basically, feel his ribs, there should only be a slight fat covering over them, you should be able to feel the spine easily and also the pelvic bones; they should also have a nice abdominal tuck from the side and an obvious waist from the top.
Here's a page that will help you understand what I am explaining:http://www.cathelp-online.com/health/bscore.php
As I said before, a vet or tech is definitely more accurate because they can actually see and feel him.
Hope this helps!!!