So, DS is 2 and has never really been a big eater. Unless it's a very very small bite of what he's eating, he will spit anything (even stuff he likes) out after chewing for a few seconds. He doesn't eat any veggies, and he only sometimes will eat fruit. And by eat, I mean chew...
He talks about tomatoes and carrots, and he likes the colors and smells of food. He also enjoys helping us make food. I'll let him "beat" the eggs at breakfast, or stir the veggies, and we do a lot of baking together. He has a blast, and then doesn't eat any of it.

Lately he's been experiencing some constipation, and I'm thinking that it is at least in part due to his diet. I'd love to get more healthy foods into him. Mamas with toddlers, how did you encourage your kid to eat?
ETA: I've been thinking about making smoothies for him with some veggies, fruit and yogurt, to have for breakfast. Has anyone had luck with smoothies?
Re: Encouraging Picky Toddler to Eat
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I also follow @ncbelle's rule of continuing to offer healthy options, even if he doesn't eat. We had pork and rice noodles last night and he ate very little of the pork. But he doesn't get something else. And it sounds like you follow a similar theory.
The only thing that raises any sort of red flag for me is the fact that usually "picky" toddlers are those that only want to eat certain things, and it sounds like your son doesn't have anything he really eats ("he will spit anything (even stuff he likes) out after chewing for a few seconds.") Does he have any issues with his jaw/teeth/mouth that would cause him pain when chewing?
Ok, there are some things he will eat. He'll eat chocolate (facepalm...thanks, Halloween) and Cheerios. And he'll eat ice cream and cookies and anything sweet. But those aren't offered every day or even more than once a week. Unless he puts too many in his mouth, he'll usually swallow goldfish and other carby things. I just don't like to keep convenience foods around, and he doesn't eat much else...
The only healthy thing I've had relative success with is cheese, but since he's constipated, I don't want to offer him too much cheese. I really want him to eat veggies and fruit, which is why I thought of the smoothies.
ETA: and you're right that we don't offer alternatives to what we make at dinner. He is offered whatever we're eating. He turns his nose up at some things and chews/spits out others, and maybe manages to eat a few bites along the way. He never seems hungry.
Apple/banana/pumpkin pancakes would be good for breakfast too and incorporate more fruit.
But overall, if he doesn't seem hungry, I wouldn't worry too much about it.
ETA: He's right on track developmentally. Has excellent fine motor skills as well as far above average speech. He speaks in complete sentences and has for some time...just thought I'd mention it in case what you're referring to is developmental.
I worked for a family with 4 kids and one of them was very similar. It was very much a sensory issue for him. It was evident even when he was 10months. Also, if the food was cut in big chunks he would just lose it, completely melt down. His twin bother loved everything big. For him I needed to cut it into very, very small pieces (like pea size). It was not a tantrum behavior he really just would get so overwhelmed and could not eat it.
What worked, and they still have to do with him at age 6 is tell him very clearly what will be for supper that day, at breakfast. Then a reminder closer to supper. When the food is presented he would be prepared mentally for it.
I'm not sure if any of this will help your son...the point being it is not always just a toddler control behavior. Some kids really, really struggle with food (for whatever reason) and they need us to find ways to help them through it. It sounds like you are doing a lot of really great things to help him with his food relationship.
Can you think about what he does eat and doesn't eat and see if there is a common thread? Is it size, texture, taste, etc. That may help narrow down the reason and help you find a way to help.