Get him trimmed, a good barefoot trim with a strong bevel will alleviate the forces that keep splitting the hoof (unless there's scar tissue at the hairline, then it will always be split, but as long as the horse is not limping it's okay, just need to be sure to stay up on the trims) . Once the "splitting" force is gone, the crack will start to grow out. If it's up to the hairline, your looking at least a year for the whole crack to grow out, but if he's not limping, he's quit usuable, physically. (who knows about his training/personatlity!)
Since he's a rescue, if there is any worm/nutrition issues, fixing that will also help his hoof heal.
Somtimes a chronic crack can indicate a fungal infection (white line disease, anyone?) but before you treat for it, try the trims for a couple of months. Then if it persists, wtih no sign of improvement ,treat accordingly for infection.