I'm not an expert, but I am second language teacher and have been raising a bilingual child for two years (our second child, honestly we were too scared after attempting for a few months with the first.) We had a similar situation where we were unsure of our commitment with our nanny and therefore had her speak in English to our firstborn...four years later, we realize we should have let go of our worries and had her speak more naturally to him (and it sure would have helped him on the playground as we were in a Russian speaking environment.)
Especially since Spanish is such a dominate language in the US, I would see having the nanny speak Spanish an asset. I fully realize the difference between Spanish and Portuguese, but there are many similarities, being a Romance language, which I would feel helpful in the acquisition of his "mother tongue" and help balance out the input since you and your husband speak English to each other and are surround (I assume) by English speakers. Furthermore, there are a multitude of opportunities to continue Spanish available in almost all areas. Yes, there are probably going to be speech delays, but hang in there, you'll be amazed when you see your child communicating in three languages.
Another thing that we found helpful with both of our children was basic ASL baby sign. It was so helpful to express needs before speech came and extremely useful in conveying the same message through different languages.
Again, I don't know the answers, but have found encouragement from others letting me know that it is possible.