I assume your girlfriend's son is a minor child and you all reside in the US. If so, he will need his passport (which you said he has) and you will need a notarized letter from his mother (assuming she's the legal custodial parent) giving you permission to travel out of the country. You should also have a letter authorizing you to act on the child's behalf in the event of a medical emergency. With those 2 documents and your own passport you should be just fine.

I would suggest you contact the airline you will be traveling on as well as the immigration dept in the Bahamas to find out directly from them what is required. The information is probably on their web sites as well.

The AIRLINE and US customs and immigration are who you need to clear. It's the US child protection laws that are in play.

Check the US gov't travel website for details.
Then make sure the airline is in agreement.

I think you also need to contact the Bahamas embassy in D.C. to find out what their requirements are. You don't want them stopping you from leaving the Bahamas because you don't have the paperwork THEY require. Don't know where you live but I believe they also have Consulates in NYC and Miami.

Not that I'm wishing you a hard time in doing this properly. But as a mother, it would really bother me to find out that someone would be able to leave the country with just a notarized letter/passport with a child with no legal/blood ties. A blood/legal parent has to have those 2 documents. I hope they require more from somewhere else.

I'm really curious to know the end result. I hope you'll let us know.