I'd go for the sanding option. What can you lose?Sand it down a bit.
Turn the board over and use the other side.probably charred meat. yes, it's like that for a long time. what can be done then??
Those crevices can harbor mold and bacteria. If he can't get it clean, I'd get rid of it.Does it smell? Did you use a brush? Plastic boards develop tiny crevasses from knife cuts. If the bleach and a stiff scrubbing does not remove it - I would not worry about it.
If it really bothers you - throw it out and buy another.
You must be really hard on your cutting boards. I have 6 cutting boards that are around 20 years old and still going strong.treat yourself to something new. i recycle mine often unless they are glass.