I have similar frustrations with my LO from time to time. Here are strategies that helped me. I should also mention that we did BLW (no spoon feeding/purees) and she is a champ at signing, so that has immensely helped with communication.
1. I only give very few bites at a time, like 2 pieces of food. If I put a lot in front of her she gets overloaded and either starts dropping food on the floor or "cleans up" and starts wiping her mat in front of her. So start with 2 bites and offer more as he requests it. This also puts them in charge of the feeding and allows them to take ownership. I offer 2-3 different things at each feeding (sounds like you do too) and again it allows her to be in charge of things and decide what she wants and doesn't want.
2. I wait till I know she's hungry. So, instead of putting her in her high chair when I THINK she should be hungry, I wait until she signs for food or even until she gets a bit fussy and I know she's hungry and usually she cleans house. From all the reading I've done, allowing them to FEEL hunger helps them associate that food ends that feeling and helps them.
3. I get really frustrated when I have cooked something nice/spent a lot of time on it and all she does is drop it on the floor. So, I stopped. She eats what we eat. And if she doesn't, then she doesn't. Basically, I stopped caring so much. I do have nursing to fall back on, but I guess what I'm saying is...they won't starve themselves (unless something is seriously wrong). Letting go of my preoccupation and obsession over how much she eats has really helped me ease my frustrations with it all. The book, "Why French Kids Eat Everything," helped me get a handle on this.
Here are my go to foods: avocado, sweet potatoes (try different flavors for variety), any type of bean (I'll make a pot and then saute a serving up with different flavors: garlic, basil, tomatoes, etc.), cheese (I use a yogurt cultured cheese as she doesn't do very well on regular cow's milk cheese), FRUIT of any kind. I also let her eat butter. Like pieces of butter (I'm able to find a great local source, but kerrygold is always an excellent choice at the market or an organic cultured butter too). Just cut up like cheese. She recently started drinking smoothies, which is awesome b/c I can hide so much great stuff in there! Full fat yogurt, kale, flax seed, nuts, anything you think they might be needing/deficient goes in the smoothie and masked with fruit and it has been a great solution for us.
Hope some of this helps!!! Keep us posted on how things are going...