Teething and grow spurts will cause this too. Has he lost all his puppy teeth yet? Gracie eats like a horse for a few days then just kind of mopes around and then I'll notice she looks a little 'bigger'. Of course define 'bigger' when you only weight 2 lbs 13 oz.
I probably wouldn't worry too much unless it goes on for many days and/or he stops eating, drinking, potty well.