This list was developed using College Navigator (feds’ website) and if I mention various majors by the school, it’s because there are a decent number of participants, per the most recent graduating class data. The amount of description by each school should not be an indication of which I think most highly of…just had more to do with my energy levels. 
Some schools you may want to consider include:
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Allegheny (PA ): Also has a large number of majors in international public health, but also international relations and affairs, community organization and advocacy, and other fields that seem to align well with your son’s interests. About 1600 undergrads.
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Bethel (MN): Majors in peace studies, international affairs, etc. About 2600 undergrads.
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Furman (SC): One of the biggest groups of Spanish majors from any of the schools, and very solid academics all-around. About 2300 undergrads.
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Gettysburg (PA ): Seconding this rec. A number of Spanish majors, various area studies, and international/globalization studies. About 2400 undergrads.
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Guilford (NC): About 1200 undergrads here
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Hamline (MN): Although there was a recent decision by the administration about an academic freedom issue that has not been well-received (additional info), I have otherwise heard good things about it and it might be a good fit for your son’s interests as it offers majors in international/globalization studies and peace and conflict resolution, as well as Spanish, all of which have some good participation by students. About 1800 undergrads.
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Juniata (PA ): About 1300 undergrads
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Lafayette (PA ): About 2700 undergrads
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Saint Joseph’s (PA ): About 4200 undergrads
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St. Olaf (MN): Seconding this school of about 3k undergrads
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Susquehanna (PA ): About 2200 undergrads
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Washington & Jefferson (PA ): About 2200 undergrads
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Wofford (SC): About 1800 undergrads