"Speech, language and sensory integration, including oral defensive issues, often go hand in hand."
I totally agree with this poster's comment. My daughter has SPD, and when she was younger we had a hard time getting her to eat. At one point, she ate lots and lots and lots of food. Then she got to the point where she'd only eat certain foods. And what she would eat wasn't considered the best of foods for a child to eat. However, we were so happy she was eating anything that we let her eat them and worked on getting her transferred over to better, more nutritous food.
Nowadays she's pretty good about trying everything once (she's 11), but she'll only eat bland food. We've been lucky in that she is very interested in organic food; she knows it's good for her and is interested in eating it. She likes comfort food, though (sweets, carbs) and we have to work very hard on making sure she doesn't overindulge. Certain textures bother her, and she'll eat some food made one way and not another.
I have a friend who's little girl refuses to eat. They took her to a specialist who "trains" children with eating disorders/problems. It's worked, but it requires continuous work and diligence by everyone.
Good luck, and I wish you lots of luck. It's a lot of work, but it's worth it and it definitely pays off. If you haven't had him tested yet for sensory issues, I would. Refusing to eat/not eating much is definitely related to sensory issues.