He really doesn't look overweight to me. Unless he's way hairier than the picture looks like, I don't see the tiny legs comment, either. He could stand to gain a little muscle, and for that I would suggest just ride him as often as possible, not necessarily fast or hard, just get him working a little, enough to wet the underside of the saddle pad is fine.
Here is a link to a short description of body condition scoring for horses. How to Condition Score Horses - by Susan Evans Garlinghouse
The pics aren't very good, but you can get an idea of how to give your own "second opinion". There are a lot of other sites and literature on this kind of scoring. It's difficult to tell from a photo alone, but he looks about moderate to moderately fleshy (5, 6), not bad at all, to me. I have seen morbidly obese horses, (as in, seriously, a score of 8-9! The size was explained to me as being massive muscle. No. ) and your horse is nowhere near the "150 lb. 5th Grader" mark.