There are six of you so I'm going to assume you're not staying on property so I'll base my suggestions on the premise that you're at a hotel on Harbor but 'close' (I'd highly recommend the Desert Inn and suites!).
Use your day wisely, get UP and do the rope drop thing each morning, the weather is cooler for everyone ....make a plan for each day to use fast pass strategy for the rides your son that uses the wheelchair might not want to ride...by that I mean take everyone's passes and send a 'runner' (the fastest walker you have) to the most important fast pass machine...let's say it's Space mountain, get the fast passes for that, put them away and get in line for Buzz - Buzz is wheelchair accessible but first thing in the morning the line should be fine. Your fast passes for space are going to mature pretty early...check when you get off of Buzz, get the next fast pass on your list. Remember you can get another fast pass as soon as the last one you got matures OR two hours which ever comes first, you just can't hold two fast passes for the same attraction using the two hour rule. Plan which area of the park you want to concentrate on each 'visit'...1st morning, say fantasyland, go ahead and get the fast pass program started while the family goes to Fantasyland. Your son can take up to 6 people in the HCA line but that doesn't mean it's a FOTL pass, it just means an alternative entrance. Make sure to go download the current Guide for guests with disabilities. After a few hours at the park go back to the hotel, cool down, rest, snack...let those who want to play in the pool if you want. wait til late afternoon/early evening and go back and have the '2nd visit' of the day with a plan...tomorrowland...if you know that you plan to go to tomorrowland when you go back, you'd concentrate on tomorrowland fast passes in the morning. There are 3 attractions with FP in tomorrowland, Space Mt. Buzz Lightyear and Autopia. To enjoy the Finding Nemo attraction just go to thru the exit and wait near where the lines are until a CM approaches you. Ask to be seated in the M.O.O. (the observation room) It's totally HCA accessible and no pass needed and we've never had more than a few minute wait with a group inside when we got there. There is room for your sons wheelchair, benches for everyone else and it's private...and FUN!