You could try an oxygen bleach in very hot water.
Perhaps laying them out in the sun will bleach them.

According to today's Consumer Reports magazine on laundry detergent testing, use Tide Ultra With Bleach powder and warmish hot or hotish warm water. It's not chlorine bleach so he doesn't need to worry. It gets laundry cleaner than any other detergent, according to this article and my own experience. Much luck. Please report your results here.
Pat

Thanks for those replies I will try some of the ideas... is there also a spray or pre wash I could use ??

I've had very good results with the Tide spray pre-treater getting almost everything out (except set-in chocolate.)

I get great results with Tide HE powder with [non-chlorine] bleach AND OxyClean.

Do you wash his undies in true hot water? IF you have an HE washer, you'll need an internal water heater to heat the water to that temp.