I am replying to your question on the basis of the limited information available on the facts of the case and without having seen the detailed documents. So, it would be advisable for you to seek opinion of some expert who has actually seen the documents.
Generally speaking, if someone is declared medically unfit for a technical post as per the rules at the time of the selection for appointment itself, then he cannot be appointed to such post. His appointment should have been challenged at that time itself, i.e., at the time of selection / appointment itself.
But, if he is senior to you and if his appointment is not challenged to be illegal, then he may maintain his seniority. The reason may be that it is his appointment which may be challenged to be illegal, and not his seniority. Either he remains in service or he does not remain in service. If he does not remain in service in view of his illegal appointment, then there is no question of seniority being decided since he is already out of service. But, if his appointment is found to be legal and/or his appointment is not challenged successfully on the ground of medically being unfit, then he would continue to be senior if he was above you.
Therefore, in my view, you may have to challenge his illegal appointment itself, and not the seniority. If he is medically unfit, then he may not have been appointed even as junior to you.
As mentioned above, this is a tentative opinion since I have not seen the detailed documents of your case. It is advisable for you to consult some expert by showing him all relevant details and all relevant rules / guidelines applicable in your organisation.
On the second part of your question, you have yourself mentioned that your organisation is not within the purview of CAT. That being the position, how can you approach CAT? You may have to approach the District Court by way of a civil suit or the High Court by way of writ petition.
Dr. Ashok Dhamija is a New Delhi based Supreme Court Advocate and author of law books. Read more about him by clicking here. List of his Forum Replies. List of his other articles. List of his Quora Answers. List of his YouTube Videos.