What is the ruling regarding the prayer of a young man whose right leg has been amputated above the knee, [praying] while seated in a chair, given his ability to stand leaning on a cane, except if the Imam prolongs the prayer?

Standing in obligatory prayer is a pillar without which the prayer is invalid. The worshipper must stand, leaning on a stick or something similar if needed, due to the general nature of the evidence.

As for one who is unable to stand for the entire rak'ah but can stand for part of it, there are two opinions regarding how they should pray: The first is that they should start seated and then stand before bowing, based on the Prophet's action during the night prayer. The second is that they should start standing, and if standing becomes difficult, they should sit; because standing is a pillar, and they might be able to complete the standing or a part of it.

Therefore, you must begin the prayer standing, leaning on a stick. If standing becomes difficult for you during the prayer, you should sit. It is not permissible for you to start seated while you are able to stand.