If the guy does good work and is trustworthy, I would keep him on. I would also be pushing him to study English and make that a priority. Work together and practice speaking as much as you can. I wouldn't leave him alone with a customer until his communication was better in case a customer had a question or concern, but have him more as a labourer to start.
Good workers are really hard to find. Good communicators are also hard to find. I personally would rather have a good worker who is a poor speaker, than a strong speaker who is a bad worker.
