Thank you for your query.
to any substance can only be confirmed by giving him a controlled challenge to the same substance. It shall be advisable to discuss with your doctor who diagnosed that he has allergies to fruits. A challenge dose of avocado and/or mayo can be given to confirm the diagnosis.
As you mention that your son is under a lot of stress
and is not willing to accept the divorce I suggest that it shall be advisable to consult a child pyschologist who can work with the child and help him in these moments of stress and anxiety.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.
Rajiv K Khandelwal