To be honest, this is the first time I've seen or heard of an applicant disqualified for optic neuritis. So I had to do a little research to get myself familiar with the disease.
I really don't know what the chances will be. You state that you have forwarded the specialists evaluations to DoDMERB, so thats a great step, and you can always have your son write a letter to the waiver authorities explaining the issue in his words, and how his recovery has been.
There are a couple of things that bother me though. Based on what I've read not all who are affected with optic neuritis regain 100% of their vision, but since you didn't mention any other disqualifications I would have to guess that your son does not have that problem. The other issue is that optic neuritis can be a precursor to multiple sclerosis. I'm sure the specialists spoke with you about this as well. Granted the number is not huge (around 35%), but I'm not sure that is a risk that the waiver authorities will want to take.
Again, I've never seen a disqualification for optic neuritis so I am putting forward my guesses. Nothing would make me happier than to be wrong, but I also believe that if there are doubts the applicants need to be aware of them. The best thing I can tell you is to hope for the best, but prepare for the worst.
If you have any other questions please feel free to ask!