You are right that nuts are not used too much here in food, maybe with the exception of some things you would get at a bakery like coconut cookies and maybe some sweet breads. Peanuts, cashews, etc are sold as snack foods but should be easy to avoid. If visiting the Caribbean coast, definitely watch out for the use of coconut milk in some of the rice dishes as well. Coconut milk could also be in some fruity cocktails??
Our advice is to write out a card in Spanish to show the server. The people here are so helpful that they will definitely make sure there are no conflicts. It would also be good to have the list of all the nuts your husband is allergic to translated. You can also ask your hotel where the nearest medical clinic or hospital is… just so that you have a sense of distance/ peace of mind.