From your post it sounds like there are two signs. One claims the right to refuse service to anyone. Which technically he can do, if he has just cause. But if he should try to refuse service based on race, creed or color he'll wind up in court, and out of business.
The other sign, near as I can tell from your post, just lets his customers know that they do not speak Spanish in his store. In an area with a large Spanish population, I can speculate that he's 'politely' letting his customers know that he does not speak Spanish, so they will have to speak English to get proper service. Regardless of his motives, the sign only says that he can't speak Spanish. No law against that. Yet.