eek - that's a bit out of my knowledge area tbh. The lactulose/mannitol test is used to test for coeliac right?
All I can say really is that children who test negative for coeliac can sometimes still improve (parental report) on a gluten free diet.
There are some data sheets you can download to record behaviours which can help you assess whether there's a difference pre and post diet. And of course there's the autism research unit which will do the IAG screening.