Without knowing the severity and scope of the types of developmental delays your son is experiencing, it's hard to give appropriate advice.
My son, who just turned 5 had sensory issues and mild developmental delays. Although as a baby he eagerly ate almost anything I fed him (as long as it didn't come from a jar or can), he began being sensitive to food textures and would not eat anything slippery such as pasta or jello. I recognized which foods caused him distress and omitted them. These foods are different than the ones he simply was being picky about eating.
Around the age of three, I knew my son was just being picky about alot of foods and wanted to eat only cereal, breads, crackers, yogurt, etc., simply because he liked the taste better. What I began doing is placing the food I wanted him to eat in front of him and had a separate dish of what he wanted, then told him to get a bite of what he wanted, he had to take a bite of what I wanted him to eat. Of course he didn't like that at all, but eventhough it was difficut and tested my patience, I was in control, not him. I would sometimes force the food in his mouth (I know THEY say not to do that), but sometimes that is the only way he learned that MY food tastes good, too. I started off with small amounts of my food vs his food and gradually altered the ratio.
Eventually, he learned that my way was the way it was going to be and he gave up fighting. I would periodically reintroduce foods that he was sensitive to and now pasta is one of his favorite meals.
Another thing, he wouldn't eat cooked veggies at the time so I gave him raw and frozen veggies. This especially worked if I fed them to him as a little "snack" while I was cooking dinner, or offered a ranch dressing to dip the veggies in.
It has been a two year stuggle, but I no longer force my son to eat, and he eats whatever is put in front of him. Even vegetables that he claims he doesn't like, the rule is each time I make it, he has to take at least one bite, since I have read that using this method, kids get used to and eventually learn to like foods they would not eat before. I have seen this work with my son...except broccoli (I'm still working on that).
I have also heard that you could give a child a few healthy choices and let the child decide what to eat of the offerings or let him give you ideas for dinner. Mine to this day would choose to have buttered bread for EVERY meal!
If these ideas are not practical for your situation, I would suggest taking to your DR (although mine hasn't offered much help in this area)
By the way, my son is gradually outgrowing alot of the tantrums, developmental and sensory issues. He was kicked out of a catholic preschool for hyper and agressive behavior (due to boredom), but has made extreme progress in both social and intellectual skills by hands-on homeschooling and lovingly strict, consistent parenting. Good luck!
K.