Light and fussy eating is SO common in this age group it could be considered normal. My grandson for awhile was 75th percentile for height and 10th for weight, so we watched him closely and coaxed him to eat as much/often as possible. There were a lot of flavors and textures he simply did not like, and all the research I did during that period suggested that the extreme pickiness is probably a survival mechanism that keeps toddlers from eating toxic things. Getting a child to eat "yuck" foods during that phase (which can last a couple of years) is considered by some developmental researchers to be roughly equivalent to handing an adult a dish of eyeballs, ants and weeds and expecting them to eat it.
While he was super-skinny, my grandboy also had a round of had-foot-mouth disease and a tummy bug or two, and lost weight alarmingly each time, but when he was well he would catch up again. He's now 5, his tastes have broadened, his appetite is sporadic but often pretty good, he eats (mostly) an excellent diet. Broccoli has become a favorite food. And he's healthy, energetic, and still quite slender.
It's so easy to worry and so hard not to, but most toddlers will make it through the "lean" and picky years just fine, L.. When you think about the seriously restricted diets children subsist on successfully in less-privileged areas of the world, it becomes obvious that our expectations here are pretty high. I'm in no way knocking good nutrition and balanced diets, but it does seem true, observing the culture at large, that probably a majority of toddlers go through a period in which they only eat a few foods.
What often works well in this age group is to allow healthy grazing throughout the day so the child can actually eat when hungry. Sometimes those little appetites fade quickly when the child is distracted by play or tiredness. Setting out a small plate of sliced cucumbers and yogurt dip, ants on a log, apple, orange or melon wedges, squares of omelet, french toast or pancake, with toothpicks or a cocktail fork to make spearing food amusing, can encourage the child to notice how his body feels and experiment with a new flavor now and then. "Parties" to which his favorite toys are invited can be compelling, too.
Try not to let your anxieties show too much. Children find this confusing at best, and a cause to resist at worst. Your child's willingness to try new foods will increase in a year or so, and watching you prepare and enjoy a variety of foods will have a more positive effect on him than insisting he eat what you prepare. Meanwhile, maybe you can invent a cookie recipe that contains spinach and squash…