Picky Eater?!

The past few days I have noticed that Gus hasn't been finishing all of his food like normal.  We are feeding him Dick Van Patten's Natural Balance and he has always seemed to do fine with it.  He is a pretty skinny boy and the vet gave us the go ahead to feed 6 cups/day twice a day.  I though, maybe that was too much and he wasn't that hungry.  I slowly cut the portions down, but each time he ate less and less.  I stated to worry...This morning I went to feed his big sister her adult food and got curious. I was desperate to see if he would eat so I put a handful on the floor and he gobbled it right up.  I followed that with a handful of his food and he snubbed it!! So...I put two cups of the adult food in is bowl and he ate it all! It was Purina One...Ugh, I hated doing that, but I needed to know he would eat.  My BF has had our black lab for 11 years and figured no need to change her food now...I am more picky about what kibble I feed Gus.  

So what now? Try something new and maybe get a small bag? Should I call the vet or be concerned at all? He did eat after all...

Any thoughts?