How high is his fusion (just in general terms whether it's neck, middle of the back, or lower back)? That definitely makes a difference on what rides may be comfortable (assuming he is cleared for all rides). It will probably make a difference on what he's medically cleared to ride, as well.
I do know people with spinal fusions who have been cleared to ride whatever they want, but definitely have him talk to his doctor before making plans. He or she might also have an opinion on things like renting an
ECV.
It's relatively common for people who would never use even a cane in their day-to-day lives need to use an ECV or wheelchair at WDW. From your description, I think the ECV is probably the better option if he does need something. It gives him more independence and he can always park it to walk through queues. It's less expensive to rent offsite, but
ECVs can be rented in the parks (just sometimes they run out of ECVs in the mornings). I'm unclear how long he'll be with you or your trip is, but if it is more than 2-3 days, you can always start the trip and then call one of the offsite vendors to get an ECV delivered to you within usually the next 12-24 hours. (It can take less than that it just depends on when you call and what they have available at that time.)
Even if he can't ride the thrill rides, there are loads of things to do at WDW. If part of your party wants to ride thrill rides, maybe you can split up so that those people go do coasters, etc. and the rest of you go with your cousin to rides and attractions he can do. That way he's not left out while everyone else goes on things (unless he wants the time alone) and you should be able to frame it so that it doesn't sound/feel like it's a split just for him.
Depending on his PTSD triggers, he may not be able to spend extended periods of time in the parks. There are just so many things going on that it can be overwhelming. I'd let him dictate the pace that you move at in general and possibly go in planning to take part of the afternoon to go back to the resort to nap/decompress and then go back (if you want) in the evening. If you make it to a park at opening, you can actually fit a fair number of rides without much wait and without rushing to every single ride. As long as you go to a park that doesn't have EMH hours, it'll also be the least crowded.
If you're used to commando touring, you probably won't be able to do it on this trip. But that really won't leave you bored. There are definitely aspects of WDW that can only be appreciated if you approach the parks at a relaxed pace.
