How long has it been going on? Is his pediatrician concerned? Why does it concern you?
My own child is a *very* light slim petite kid - (31-34 lbs in 2nd grade) We worried and stressed but came to accept that he is a small kid (we were small as children ourselves, so why should we expect him to be big?!)
What we have learned about WHEN to be concerned: If he suddenly starts losing weight, for no explained reason - that's when you get him checked out.
If he's holding steady on his weight, just not interested at the moment in food, he's probably "feeding" more voraciously on experiences and taking in all that his world has to offer.
Another thing to consider is the growth rate of babies. When they are weeks and months old, they grow - A LOT. They double their weight. But around the time they hit a year old/start walking, they slow down. If your year-old kid continued to gain at the rate of an infant - that's trouble!!
So, enjoy his "other" ways of growing and all its ups and downs.
My son would eat a lot and I'd be happy, thinking "finally! he has an appetite! he's eating as he should!" then seem to stop eating (but not really, he just ate "just enough" which was not nearly enough for ME) and I realized he was on a growth spurt. They do eat more for a little while, then stop and then you see that they've grown - either physically or developmentally or both. It's a fascinating process!
So he may be coming off a recent growth spurt to where he's "just not that into" food at the moment but he will eat again. And again. And again. and when you're a teen, you'll wonder why you ever thought his eating just a little bit was a problem! ;-)
and yes, I agree, try different things. Give him tastes off your plate. Also, snacking continuously makes him not hungry enough for a meal. Feed him 5-6 times a day (breakfast, mid morning snack, lunch, afternoon snack, dinner, maybe bedtime snack. All about 2-3 hours apart)