T.F.
He can learn both languages now. At 15 months he won't have a large vocabulary in either language. Boys generally speak later, but make more noises and sounds, like truck noises, so it's great that he already has some Spanish vocabulary. Children who are raised in a bilingual home often speak later, but understand both languages and know the difference between the two. If he is hearing Spanish at home and English at school and shows understanding, I would just be patient and let him learn. The research I've read says they learn two or three languages more easily the younger they are because of they way their brains are developing, so the advice to "wait" is not valid. I've also read that it's not necessary for one parent to speak each language, you can both interchange, just not in the same conversation.