I don't know what the recovery is like from ACL surgery, so his doctor may not want him to ride "thrill rides," but there's a lot at WDW that he should be able to do just fine.
Most of the lines are accessible, and ones that aren't will have another option. Cast members will always be happy to help you figure out where to go, and there's a guide for guests with disabilities available at guest relations, as well as in PDF format somewhere on the internet, that spells out the requirements for every ride and attraction.
Whether you decide to use a
scooter or a wheelchair for him, rent one from an outside company. They will deliver to your resort. Disney's "preferred" companies can leave the rental (this is assuming you're staying on property) for you to pick up, other companies do require that you meet them to pick it up. It can be a hike to get around your resort or hotel, and from the bus stops/parking lots to the parks, so having the mobility device at those times helps a lot too.
If you plan to drive to and from the parks, most of the scooter rental companies have models that will break down for transport in a car trunk.
As for the crutches, I think there are a couple rental companies that will even put a carrier on the wheelchair or scooter for him to store the crutches in, so he can use them to transfer or move short distances.
Also, if you're flying, don't forget that you can ask for wheelchair assistance in airports too. There's no reason he should have to limp through airports on crutches!