Hi Welcome to Fish Lore

I'm really sorry your pleco isn't feeling well. He's lived a good long time with you.
Have you checked your water parameters, is so what are the readings for
ammonia nitrites and nitrates? Have you added anything new to the tank recently? Is there anything he could have hurt himself on?
Sorry for all the questions, the answers might help the members figure out what's going on with him.
Also, Fish Lore is a public forum and easily accessed through search engines. For your privac, you might want to consider changing your user name to something other than your e-mail.
Here's the link:
How Do I: Request NEW UserName
Good luck, I hope he improves quickly.