Do not boil the skulls, just simmer or warm water. Also if you add peroxide or oxyclean to the water, be careful as it will also whiten the antlers. Dark or yellow is still grease in the skull. The longer you wait before cleaning the skulls, the longer it takes to degrease as in the case where your sons was shot in September vs yours being more recent.
If all the flesh is off, degrease using some dawn dishsoap in warm water and keep changing the water. It is ok for the skull to sit in the water for extended time. When degreased, you can then whiten it with either adding a high concentration peroxide to the water or take the skull out and apply a peroxide/whitening powder then wrap in saran wrap or a bag for a few days.