First contact the airline and get their OK, they may ask for a fitness to fly certificate from his doctor. The doctor may charge a fee for doing that. They may allocate a certain seat for him so he can be more comfortable.
You should also contact the hospital where he was treated or will have his follow-up and get their OK. Explain the situation and ask whether he needs another cast put on. Sometimes they will substitute a lightweight cast which he would probably find more comfortable or even a removeable one. There can be a problem with swelling on long haul flights and circulatory problems if the limb swells inside the cast.
Have you told your travel insurer? They should be informed so that any complications are covered if he does travel.
Once you have all the facts you can make the decision whether he will travel or not, but more than likely it will be fine. He can easily hire a wheelchair in the parks and you can ask for a wheelchair at the airport too if it would help.
Wishing him all the best.