Is there Janazah Salah on a missing person?

The obligation to perform ṣalāt al-janāzah sets in when it becomes known that a person has died.

When it is merely suspected that he might have died, without specific evidence to support that suspicion, the obligation to perform ṣalāt al-janāzah does not set in.

Our Fuqahā have mentioned numerous aspects pertaining to missing persons, such as their property, marriages, obligations and rights. However, despite the great amount of detail given to these various aspects, and notwithstanding the fact that people have gone missing throughout history, not a single one of our jurists is on record as having prescribed ṣalāt al-janāzah for a missing person.

Therefore, under the circumstances you describe, there is no obligation to perform ṣalāt al-janāzah over your missing uncle.

Also, the performance of ṣalāt al-janāzah under these circumstances would not be valid.

والله تعالى أعلم

And Allah knows best.