Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. So sorry to hear that Lenny has MG, but understand completely your concern and desire to do everything possible for him.
Your vet is probably gradually increasing the pyridostimine (the main treatment for MG), to determine Lenny's correct dose and to avoid overdosing him.
MG is an autoimmune disease that affects the acetylcholine myoneural receptors causing the weakness that Lenny had. Because it is an autoimmune disease it is possible to also treat this with immunosuppressant drugs like prednisone. However, MG can lead to aspiration pneumonia, so prednisone could be dangerous to give.
It is important that Lenny be receiving a well balanced diet, as always, and if he has megaesophagus with his MG, then feeding him in an upright position, small meals several times daily is the treatment for that.
If he is on a high quality dog food, then additional vitamins aren't necessary. Omega 3 fatty acids are good anti oxidants and therefore anti inflammatory, these may be of some help to Lenny. These are also used commonly for kidney disease and arthritis in dogs, and your vet will have some on his shelf.
I think a very informative web site for pet owners is www.veterinarypartner.com. Go there, click on "dogs" on the left, and then put myasthenia gravis in the search bar.
Sure hope this information has been helpful. Sounds to me like your vet is treating Lenny very appropriately, and adjusting his meds according to his needs. If I can be of any further assistance, please let me know.
Or. . .I should say if you have any further questions that I haven't addressed!. . .