Personally, I think a business major is more employable than a PolySci.
With that said, I'd wait to see if he can get a job offer before he graduates with the PolySci, and if not, stay the extra term and get the Business major as well.
If he is interested in business, I'd suggest he apply for jobs, even if they do require a business major in their listings. With his minor & experiences, they'll consider him probably.
Congrats on the internships. Business internships usually are paid (not sure about PolySci ones) but are def competitive.