What can I do if my baby is deathly allergic to cashews and pistachios, does this mean he is allergic to mangos?

From Allergy Advisor. One would expect an extensive cross-reactivity among the different individual species of the genus (e.g., mango, cashew and pistachio nut), but this, in fact, does not occur frequently(27). Cross-reactivity has been found among pistachio nut, cashew nut, and mango seed, but this cross-reactivity did not extend to mango pulp.
Not necessarily . Mango fruit and nuts are different. It is not automatic that your child will be allergic to mangos. However mango fruit can cause allergic reaction.