I don't know a lot about Autism, admitedly, but I do know that sufferers of the condition are often very easily unsettled by changes to their routine.

I would have thought that it's possible that your grandsons tantrums could be caused by the change to his routine rather than his diet when he's at his Dad's.

As a child of divorced parents, I know how easy it is for one parent to blame the other when things go wrong with the kids, but it's not always as black and white as you may think. You have no reason to believe that his father would sabotage your grandson's diet deliberately, and no actual proof that he has (as Lil points out, the things he's reportedly eating are ok for a wheat and dairy free diet) so perhaps you should be giving your ex son-in-law the benefit of the doubt, rather than jumping to conclusions before you know all the facts 1st hand.

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