Have you already been to a pediatric Occupational therapist?
That's a great place to start. You may have to try a few before you get a good fit (so what else is new?)
Cheaper if your insurance doesn't cover it, is to try the 'Transforming the Difficult Child Workbook' by Lisa Bravo approach. First you notice everytime he shows the smallest hint of flexibility and adventurousness away from the kitchen. Wait for him to praise himself for his adventurousness and for 'going with the flow' Then start praising those same behaviors in the kitchen. If that doesn't work you'll have to read the book and teach him how to do a 'Reset' when he breaks a rule.
Look on the bright side - you are doing great a great job not making a fuss over his current behavior. That means it's likely not an attention-getting bid. So that leaves an unusually large biologically based hurdle or a lack of 'inner fortitude' to overcome the normal sized hurdles that everyone has with certain foods.
Is he loosing weight? Falling off his growth chart line? Having hunger based meltdowns? If not you can just keep going the way you've been. I think it's hard for adult-eyes to have a feel for how much is enough for little ones.
Shrugs and more shrugs,
Grinity
