I can understand your frustration! Looks like your husband doesn’t really see what a fantastic advantage growing up with two or more languages could be for your child. A few things, like books on the subject, might help (this way the information wouldn’t come from you, but from a neutral source, and he might be more receptive to it). Also, don’t give up looking for other families that are in the same situation as you and whose children are learning and speaking portuguese. Check all possible expat pages for your city (FB, expat sites, there are lots of resouces on the internet), the embassy might also be able to help you. If your husband met others who have made it work, he might be motivated to do the same with your child. If not, at least your son could get exposure to the language through them (playdates, even playgroups in the language?).
Other ideas (just throwing them out there, in case some might work) – how about an au pair from Portugal (or family members who could visit – maybe they could help convince your husband?), or alternatively a babysitter (check sittercity.com for your city, they might have portuguese speaking babysitters). A great site for portuguese is http://www.sonia-portuguese.com/. It has lots of resources, also for portuguese from Portugal. Other sites that you might want to check out to look for other portuguese speakers (for ideas or maybe even to locate some close to where you live) are the http://www.multilingualliving.com and http://www.incultureparent.com. Both have also Fb pages that are very active.
all the best (don’t give up!)