First locate and repair the leak, oil level must be maintained between low and high marks on the bottle. Once you are done with that then change the steering oil completely.
Try to consult some other mechanics as well, as this one is demanding a high amount. Though, it is true most of the mechanics try to avoid repairing steering gear box assembly as it is a tedious job and needs high level of expertise so if you doubt skills of your mechanic and at the end you end up buying a new (junk spares) steering gear box assembly then it's better to get it in the first place.