The first time we went to WDW with DD's first wheelchair, we were really worried about it because it cost nearly $4000 and she would not be able to use a rental stroller if someone took it. We took a lock along, but never ended up using it. You can lock two wheels of a stroller or wheelchair together so it can't be moved easily. If you lock together either the front 2 or rear 2 tires, it can still be moved by 'popping' it up on the unlocked tires. But, no one will be able to use it. That is OK to do; you just can't lock it to anything and it does still need to be able be moved if necessary.
For wheelchairs and strollers that are being brought into lines (which you probably will be doing), your son will either be in it (like for shows) or in most rides, it will be parked where you left it when you got on.
Most people don't even see the wheelchairs and special needs strollers that are parked there. I can tell you that from personal experience from last October. DH, DD and I went on Haunted Mansion. I was in the ride car in front of DD and DH. DD's wheelchair was parked at the ride exit. When the ride ended, I got off, but DH and DD stayed on for another trip. By the time I realized they were not getting off, it was too late for me to get on again. The CM told me to just wait at the exit for them to come around again.
I was a little self conscious and thought everyone would look at me when they exited the ride. Even though I was standing there for one whole ride circuit, no one glanced at me or at the couple of wheelchairs that were also at the exit. Most people are just focused on getting off, getting their party all together and getting to their next attraction.
I stopped being nervous about leaving DD's wheelchair a long time ago, but after my experience at Haunted Mansion, I definately won't worry about it any nmore.