Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Without being able to physcially see him, I am sure you can appreciate that it is difficult for me to tell if he is dead or alive at this stage in our conversation. I do suspect he is likely alive (since he's not floating at the top of the water column) but is bottom sitting due to being very unwell (potentially there i an ongoing nitrate toxicity or he has a bacterial blood infection)l.
If you wish to determine if he is alive, then you want to look for gill movement or consider giving him a gentle wee poke with a blunt object. This often will get even bottom sitting fish moving and confirm life. From there, you will want to relocate this fish into a hospitalization tank, examine him and let me know your findings (any lesions, red lines on the fins/tail, distended belly, etc), and potentially test your water parameters (ph, ammonia, nitrites, nitrates).
If you can do that and reply to me, we can discuss potential causes and an approach to treatment. Or if he is nearly dead when you do handle him, let me know and I can outline how to prevent him from suffering further.
Speak to you soon,