I'm afraid I've not read all the replies, so sorry if I repeat anything anyone else has said!
Getting a set of wheels would be a great help for you guys. You might want to consider getting a kids wheelchair (or medical stroller) with safe tie-down points. This will mean that he can stay in the wheelchair on the bus, rather than having to transfer out. I used to transfer and fold my wheelchair (back when I had a folding one, I've got a rigid frame now), but it always caused me a lot of pain and cost energy. After one particularly bad trip, where I got jostled and bumped so badly by the crowds that I was almost in tears with pain, I decided to try staying in my chair - it was soooooo much less painful! I now do this all the time and it can give me up to an extra hour in the parks, over what I would have had if I transfered.
You might also want to consider using the wheelchair on rides, where possible, as well. I use the wheelchair car whenever one is available, to cut down on the number of transfers I have to make. Again, doing this adds hours onto my stamina, and keeps the pain levels under control. Also, all of the parades and shows have wheelchair viewing areas, that will let him stay in his chair and still see all the action. For parades and Illuminations, the viewing areas are marked on the regular park maps.
Talk to him about this before you book your wheelchair / stroller. It may be that he'll feel embarressed being seen in a stroller; or that he thinks a stroller will draw less attention than a wheelchair. Just discuss the options with him and his doc, and see what everyone is happy with (also, check the height of the push-handles, so you don't hurt your back pushing him). Hopefully he'll be so busy enjoying himself, that he won't care what anyone else thinks! A good foam cushion will go a long way to making it more comfortable for him.
Also, make sure that whatever you get, you decorate it up! I always cover my wheelchair in beads, ribbons, etc for Disney, and it always gets me lots of positive comments. A pirate flag might be a good addition for an 8 year old boy

Lights are particularly useful for night-time entertainment, because not everyone is looking at that height when they're walking. The light-up carts that sell things at all the parks have LED necklaces, which can be wrapped around the frame or go through spokes.
You shouldn't have too many problems with lines, as January is supposed to be a fairly quiet time of year. If there is a particularly long line for a ride, you can use the Fastpass system to avoid the wait. I don't know if you're familiar with this or not, but it's a ticket with a time window printed on it. Just go back to the ride during that window, and you go through a much shorter line and get on quickly!
You'll have to play your touring by ear to some extent, and be prepared to stop what you're doing and head straight back to the hotel / to the first aid station if he needs a rest. You might want to make a 'must do' list for all the family, of all the attractions you really want to do. That way, even if you don't get every single ride done, everyone goes away happy.
With fatigue issues, you need to make sure he takes extra care of himself at the parks. Just a few random tips:

Don't spend energy you don't have to - plan your route around the park to be logical, and try to be near the restaurant / Fastpass ride you're aiming for when you're ready for it

Keep him well hydrated (you might want to carry a plastic cup or bottle with you, as I always find water fountains hard to use from a wheelchair - I usually end up with more down my front than in my mouth

)

Eat regularly, and maybe carry a few small snacks with you, to boost him up when needed

Keep cool - take regular breaks in the air-con, even if it's just for a five minute browse in a store

Try to protect his immune system as much as possible, as fatigue often weakens it a bit. As long as you wash hands regularly, especially before eating (or use antibacterial hand gel), you shouldn't need to worry.
I'm sure you guys will have a wonderful time, and hopefully the sunshine and Disney magic will help keep him going (it certainly does for me!)
