It's not as much walking as WDW but there's still going to be plenty of walking and lots of standing in lines. If it were me I would consider renting him an electric wheelchair and getting a doctors note of form and getting him a GAC from guest services.
No rental company is going to rent an electric wheelchair to someone who is not experienced at driving one. We tried to rent one for my mom once and was told that since they are so much harder to control than an
ECV, that unless she was used to driving one at home they would not rent one to her. If you are talking about an ECV, almost no company will rent one to anyone less than 18 y/o old. I know we rent an ECV for my mom when we take her down there and have rented from both Apple Scooters and Walker's Mobility. Both have it posted on their sites that they will not rent to anyone under the age of 18 y/o. Here is the comment I just copied and pasted from Apple's website:
Rentals are available to qualified customers only. Minimum drivers age is 18 years old. Rentals are subject to availability.
If the child is used to driving an ECV and has one at home that he uses, Randy's will sometimes rent to someone younger than age 18 y/o, but they deliver the unit and observe the customer's ability to use it. As far as I know though, Randy's is the only rental company that will even consider renting to someone under the age of 18 y/o and then only to a kid that is experienced with one. Really the only option for the OP is to rent a wheelchair, that the parents push. Depending on how big the child is, they may need a pediatric wheelchair as the adult wheelchair's seat may be too big for him and cut into his knees making it very uncomfortable.
Also you do not need a doctor's note for a GAC and in fact the CMs will not even look at it. Due to HIPPA laws, they are not allowed to ask about a disability and only want to know what accommodations the child might need. If it's just a walking/endurance thing their recommendation will just be to get a wheelchair for him.
OP: Even though Universal does not have as much walking as WDW, I would think it would still be too much for your son. There is no use him being in any more pain that he has to be. I'd rent him a wheelchair from offsite. They will ask for your son's height and weight and can make sure he has a chair that he can be comfortable in. If he feels like walking, he'll still be able to and may feel better having the chair to lean on, as he pushes it. He could leave it with the strollers and walk onto rides if he wanted. At Forbidden Journey, my mom had to transfer from her ECV into a wheelchair (since
ECVs are not allowed in that line). She got to see part of the queue, but then they had us get into an elevator, so we missed part of the queu.
Just FYI: due to several arthritis my mom has terrible knees (needs surgery but refuses to have it). There is no way she could do Universal without a mobility device. If she couldn't have an ECV, we'd be pushing her in a wheelchair. Both at WDW and Universal when we've asked about a GAC, their response has always been the the GAC is for hidden disabilities and if the person is in a W/C or has an ECV that's all they need as long as there is not some other hidden disability that might need a different accommodation (autism, poor vision, etc). Their response to endurance or mobility issues is to rent an ECV or W/C.