Some unique mechanic with Skeleton in relation to bones (like how currently Skeleton can donate health in place of bones to open the Aquatic Route)
One possibility is that bones increase Skeleton's max hp based on how many he is carrying (up to 8 max hp, not including rhino skin). So 4 Bones would max it out, unless his hp is reduced by Crown of Death, in which case 5 (this is assuming that it would be 1 bone per max hp).
Another possibility is that bones could function like an overheal/shield for Skeleton, losing a bone instead of taking damage.