I suggest checking out dogfoodanalysis.com for some reviews to help pick a good quality food. To be honest, Purina, Pedigree, Iams, Science Diet, any of that stuff; is pretty low quality; despite their awesome advertising. So I'd suggest switching to a better food that's grain-free; Taste of The Wild is a good, affordable 6-star brand.
And, some dogs free-feed well. You say he doesn't seem to eat a lot; but is he maintaining his weight okay? As long as he doesn't appear to be losing weight; it's safe to say he's maintaining it well. You should start measuring the amount you leave out vs. how long it takes to get back to empty. That'll give you an exact idea of how much he's actually eating; so you can compare to the recommended amount for his weight on the bag.