It`s good that the vet has ruled out a connection with the Advantage and they have come up with a diagnosis of what is wrong. Hopefully, he is already feeling a little better and once he is home and getting all the love and attention only you can give I am sure he will soon get back to full health.
Get well soon sweetie Minion, prayers and positive thoughts going out for you and your loving miowmie.

love and hugs
Chris