Oh boy, get him some benadryl that is what we use on our dog's when they get any allergic reaction. Get the baby one and follow by weight. The skin on Mango's is poisionous, not deadly but will more then likely flare your face up, alot of people don't know this, you have to be extra careful how you handle the eatable part and make sure that the outside of the skin doesn't touch it.
I would call your vet ASAP. he may have an allergy. Benedryl may help, but he may need something stronger. Plus, you should call your vet anyway before administering Benedryl just to get the right dosage.