Maddy was the same 'til she was over 2 years, just leave him to fill out at hs own pace, he looks fine.

I have known several Boxers who were very lean in growing on through puppyhood/into adolescence - some boxers dont fill out until 2 or 3 yrs of age, but each one of them was as fit as a butchers dog, well muscled and glowing with health.
Unfortunately some people think unless a dog is rolling in blubber, there must be something amiss.
Dogs should be FIT not FAT!

Off to try and take pictures of my skinnydog and fatpuppy!

This woman sorry don't know much about boxers.Boxers should not be over weight,your dog sounds fine to me with what you feed him.You are suppose to see a certain amount of the rib cage,usually the first 4 ribs.Too much weight on boxers will give them breathing problems.But you will probly know more than me.Take no notice, do as your doing.