It may be a mango allergy, or a sensitivity to milky sap, which is very common. It is similar to a latex allergy.
I break out in a blistering rash any time ANY milky sap (mango, plumeria, gopher purge, hoya etc) gets on my skin. It can spread past the initial point of contact too.
Over the counter steroid creams and oral antihistamines help. If the rash spreads or gets blisters, he should go to the doctor.
Carolyn