What does the doctor say? That is the most important thing. This is not the best place to get what amounts to legal advice that could have significant ramifications. If it were me, I would ask the doctor if going to WDW is okay and if so what limitations should be followed (i.e. wheel chair, frequent breaks, etc.) then I would check with a lawyer (if he is in a union, he should be able to get a legal opinion for no cost).
Where there are only three days I would err on the side of caution and postpone the trip, unless I was able to get all my ducks in a row real quick. Worker's comp insurance companies can be brutal. The laws affecting coverage and on what basis coverage can be denied varies from state to state. Even if your right, it is a risk.
I know the worker's compensation company my employer uses is hideous and denies every claim no matter what, you then have to fight for coverage. Luckily we have a strong union to help. For example, I am a Firefighter/EMT and after transporting a patient to the hospital, was notified that I had been exposed to meningitis. I was ordered by my Captain to go to the hospital for prophylactic antbiotics. I missed no time but had the the bill for treatment. Worker's comp denied the claim and the union had to fight it for me. In my job people get hurt and in 22 years every single claim I know of has been initially denied and had to be fought for. I am sure this works out well for the company particuarly when there is no union for protection, many people can't afford to fight these battles on their own especially with no income due to injury.
Good luck, I hope he feels better soon and is able to get back to work and play.