[thejeff]

If he's diseased, giving him regen will keep him from losing hits.
There are a number of ways to do that:
You could give him a ring of regen or other regen item.
Since you have a nature bless, you could make him prophet or give him a Shroud.

If he's already sacred, you can take into battle and bless him. If not you could cast a spell granting regen on him during a battle. Either way, he'll heal back up to full during the battle.

None of these will actually get rid of the disease. For that you need healers, Gift of Health or the Chalice. Healers won't work on Old Age afflictions (or on undead)