Oh how scary for the both of you! I hope that the it's nothing serious, just the flu or something like that.

My dad had something odd like that a year and a half ago. He had numbness/weakness in his arms and legs, then his joints swelled up, and he had to have fluid removed. There was a rash as well, and a fever. The doctors had no idea what it was. They ran so many tests, but nothing was conclusive. He wasn't able to work in his garden, go fishing or anything. The only thing anyone could think of was that it was auto-immune. He hasn't had a recurrence to date that I know of, but it lasted for months.

Lots of healing thoughts and prayers going out for your son.