We've had a similar issue with our 16 month old daughter. Around about a year, she started getting picky. I was concerned, naturally. I talked to some friends, and our doctor and everyone told me the same thing. 1) This is very classic toddler behavior. They are way more interested in exploring than eating. 2) Just keep offering nutritious choices and he will eat enough.
I think a really important thing to remember is do not make a big deal of how much he eats, don't play games, don't coerce him into eating, and don't try to shove one more bite in. Don't fight about it with your husband in front of him or get stressed in front of him. I say that because if you do, what you will get is an older toddler who withholds eating to assert his independence. What we did is put the food out, offer it to her, and we ate with her. When she started to loose interest, which we knew by a) she'd throw the food on the floor, or b) use her arm like a windshield wiper and toss food everywhere by doing so, I'd say "Are you done? Are you all done?" and put my arms up on the air. she quickly caught on, and now when she's done eating she will throw her arms up in the air and say "daahhh!" Which is done. I say, okay, you're done with dinner. That is that. Now, here is something else. I have tried a virtual smorgasbord of foods, and there are only a handful of things she consistently eats. I've been told it can take up to 15 tries of the same thing to really determine if they like a food or not because their tastebuds need to get used to it. Her new sitter, however, has tried every food I have, and my daughter eats every bite. Go figure! Just keep trying, and try not to stress. If he is hydrated, alert, happy...he's fine. Oh, and toddlers will tend to slow down their weight gain and thin out a bit around now. Don't let that worry you either. If you really become concerned, take him to the doc for peace of mind. Hope this helps!