This sounds a lot like a texture thing. I gag - STILL! - on cooked peas and lima beans, although I can eat peas raw now. Some kids just can't cope with this or that texture and it really truly does make them gag.
The good news is that some kids outgrow this. Natalie is pushing 5 and it's just in the last month or two that she's started eating fruits with "specks" (her word for seeds or pits or texture or any kind, really), so she can NOW eat strawberries and blueberries. She's still not fond of potatoes, but a couple months ago consented to eat scrambled eggs and onions and a couple weeks ago added mushrooms to her personal diet after years of passing on them. Crunchy peanut butter (actually, ANY peanut butter!) was right out, but now she ASKS for it. We used to joke that if it came from a plant she wouldn't bother, but she's expanded a great deal. Josie is 8YO now and while she drank rice milk as a toddler, she has actually developed an aversion to the ever-so-slight powdery texture of it. I think we could get her used to it if we worked at it tho.
There is a thread someplace, maybe Growth and Development, on Sensory Processing Disorder. Basically, that's when one is oversensitive or undersensitive to some sensory input, whether it's visual, tactile, auditory, whatever. It's not uncommon for kids to have texture aversions.
One book you can look at that has some neat ideas on helping kids get past sensory issues is The Out-Of-Sync Child Has Fun b Carol Stock Kranowitz. One thing I remember from the book was to have a sort of "theme meal" where kids eat things that are, say, round, or yellow, or whatever; they sometimes get so wrapped up in the experience that they are able to eat without the reflexes kicking in. Might be worth a look; your library may have a copy.
Meanwhile, I'd work on things like banana bread and carrot cake. Maybe serve a grape-blueberry salad, or serve blueberry juice for now instead of the actual berry, or maybe buy berries and have him help you cook them down with sugar and mash them into preserves.