It seems that in doing this I've opened some pores in the surface, which I'm concerned might be able to hide food contaminants.
If you're thinking of bacteria, it's probably not as great a concern as you might think. There's reason to believe that wooden cutting boards are as safe or safer than plastic cutting boards, and that the porosity of wood is actually an advantage.
Also, while your cutting board probably looked perfectly smooth when you got it home, it's very reason for existing is to meet the sharp edge of a knife. If sanding opened up some pores in the surface, then repeated cutting with a sharp knife would surely do so too.
I was wondering if there is a way to close these pores?
The usual thing to do is to oil the board using a food-grade oil that won't turn rancid. Mineral oil is a common choice: it works well, and it's cheap and readily available. Some people like to use beeswax along with the mineral oil. None of this will actually close all the pores, although it may fill them to some degree. Oil or oil and wax will help to condition and protect the board, especially when you wash it in warm soapy water. Re-apply the oil from time to time.