If he isn't married, try to snatch a princess from somewhere and hope he lives long enough for his son to mature.
If he is married, hope for the best. Maybe his wife might give him a son.

The only alternative that I can think of is engineering a civil war and backing the rebels. It will cause a major setback for your sultanate (or whatever it's called) but at least the chances are good the leader of the revolt will be fertile.