K.,
My daughter was diagnosed with Sensory Integration Disorder when she was 8 months, in part, because I am a Physical Therapist and could identify what I was seeing. Since then, it has taken us on a long journey.
First, I would agree with a lot of other posts. You need to have your son evaluated to get an accurate diagnosis so that accurate treatment can take place. It is better to know what you are dealing with and go from there.
Second, your son is actually eating a pretty good variety. For example, we were on the "bacon and muffin" diet for a year. I learned that my child COULD, in fact, starve to death after going from the 50% percentile in weight to off the chart (less than 3% on the weight chart). She gained less than a pound between age 2 and 3. I found a nutritionist that is superb, works only with kids that have these issues, and has even recovered her own child from such difficulties. She directed me to a pediatrician that also works with these types of kids. We drive 45 minutes to see him, and it is worth it. My daughter was put on a medical food and we have since increased her weight gain to close to 10% on the chart.
I know a lot about this topic. If you'd like to contact me to discuss nutritionists or doctors, books, or recipes, I'd be glad to help.
One more thing....in an effort to give my daughter more nutrition, I developed a healthy, natural product that you can add to the things HE ALREADY EATS. If you are interested, just let me know.
Good luck.
R.