BobS:
practice the blending. You're only staining the knot - not the
surrounding wood so you'll be trying to make it lighter looking

Good idea, if the knots will take any stain. There are people who paint
fake woodgrain, why not paint out knots.

What about bleaching the knots? He could try a thinned down bleach or
peroxide on just the knots. Test it on scrap first.

I know someone who has a knotty (not naughty) cedar wall and it looks
great. The OP might want to give it a few weeks and see if he grows to
like the knots.
--
Mac Cool