Oh, dear - I hate splinters. I had one under my finger nail just the other day (dh had to get it out).
What have you tried? As horrible as this is, one of you will probably have to hold ds while the other extracts the splinter - it's amazing how you suddenly feel fine (after being in loads of pain) once the splinter is gone.
When I've gotten splinters out, I've always pushed up under the splinter - so you are pushing it out of the hand. And I always have a pair of very fine tweezers on hand so that as SOON as possible, you can pull with the tweezers (which doesn't hurt, as opposed to pushing the splinter up, which KILLS).
If you can't do it, I would imagine you'll need to take him to see a nurse and let them get the splinter out.
I'm so sorry for your little one (and for you, having to watch him suffer/cause more temporary suffering).
I'm probably not telling you anything you don't already know...will be thinking about you and ds...