Undiagnosed. Healthy father developed problems after dental implants

Hello. I am writing to you in regards to my father. He was a year ago a very healthy 73 year old with only a history of high Cholesterol. He is very active, plays 3 or 4 different sports. A year ago he underwent dental implants and shortly after developed a respiratory virus and a very horse voice. He took many different antibiotics and it resolved. Shortly after that he began slurring his speech to the point that we thought he had a stroke. He had a MRI and it was negative for anything. The slurring has gotten way worse, can barely understand him, and he also has a lot of trouble eating. He has had 2 MRI's, and a Endoscopy which lead to the removal of polps on his vocal cords (deemed not related), multiple blood tests which have been negative. His symptoms are similar to myasthenia gravis but this has been ruled out. We also did a metal allergy test due to the implants which was negative. At this point we are not sure what to do. Can you confirm if you have heard of this and if we made an appointment at the mayo clinic they would be able to help us? We live in Louisiana. Please let me know if you can assist. Thank you very much!