I hate field day.
What I do in these cases is to just be honest with him, to explain that he has MD, and that it makes it more difficult for him to do things like run races. That he's very courageous for wanting to, since it's harder for him. I think that explaining and talking about why it's hard and how it's not his fault make it easier for my son - I don't know if that's helpful for you. Then we cry together a little bit, and then we make a list of things that might make us feel better (a new book, going to the zoo, making cookies, etc).
I'm so sorry. :( We just found out today that my 8 year old son has Field Day tomorrow. I was going to have him skip it, but he's excited. His teacher signed him up for a group tug-of-war and 3 legged race. I have to work but my husband will probably drop by and observe to make sure all goes well. The school made it seem like it's a lot of different stuff and they'll be playing games in between, so no one will really notice if he's only doing 2 'events'.
I would seriously consider purchasing a scooter for you son, not for regular use, but for use in these sorts of events.
Other kids LOVE the machine, are envious even. It will allow him to participate at "regular" speed. It can be very liberating. In my experience most kids don't care if it is their own legs or the machine that lets them participate.
In hindsight I wish I would have done the same when my son was starting elementary school.
We tried one year of field day with similar encouragement. I think it depends on your son's temperament, but it was definitely heart breaking to watch our son struggle and feel so badly. The solution that works for our family now is to take a family field trip to a local museum on that day. We have done this for the past three years, and now really look forward to this day each year!