La mayoría and son

Hola Jos

Generally, partitives like "la mayoría de..., la mitad de..., un tercio de..., etc admit agreement in plural and in singular, depending on what the speaker is considering as the "nucleous" of the subject. However, when we have the verb ser/estar and an adjective (which is this case) the agreement is in plural, hence "la mayoría de los sustantivos son masculinos" (not es masculino).

Here is an explanation about his (in Spanish though) 

I hope it clarified it.

Saludos

Inma