Fructose intolerance isn't just fruit, it's a type of sugar that's found in lots of different foods. I know someone with this intolerance and they have to be very very careful about what they eat, too much glucose can also bring back symptoms, there needs to be a balance between glucose and fructose. I'd suggest talking to a naturopath or someone like that to see where to start. A second paediatrician would be ideal though, although I understand it must be hard to do.

Good luck, I hope you get some answers soon.