This question is not easy to answer precisely without knowing how old your child is. However, I can offer some general comments.
First, if your child is still a toddler then I would not worry
too much as long as his ability in Spanish is fine (if he has limited language development in Spanish as well, that could suggest a language delay or hearing problem). However, as long as his Spanish is ok, then I suspect that his English will eventually catch up. There is data that shows that children raised in a multilingual environment may initially acquire the individual languages a bit more slowly, but they usually catch up just fine by the time that it matters.
On the other hand, if your child is already in school and having problems with English that is another matter. Still, his capacity to learn English is probably fine, but sometimes children with limited English proficiency have trouble in school, either because they are picked on by their peers or because they have a hard time understanding the teacher. In that case, it would be worth your while to talk to his teacher and see what can be done.
Talk to your pediatrician
for more information.