also, if you "hand feed" the villager you can get ticks back on his health bar, but it takes a lot of time, and a lot of feedings - although it is useful in emergency type situations

also, as VI said, move him to farming where he cannot injure himself as often - after a while he will naturally begin feeling better!
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Why is it that if someone tells you that there are 1 billion stars in the universe you will believe them,
but if they tell you a wall has wet paint you will have to touch it to be sure?