We are very precise as to (repetition is always a good idea) exactly what our needs are: bulkhead seating, and being specific about a two-person full lift from w/c to aisle chair, and into seat. We tell them when we check in, and again when we get to the gate. We make it clear that my husband is paralyzed (don't assume that they will figure this out). We explain how any other than bulkhead doesn't work, as those long legs completely block the inner seats. We make sure they understand that we must sit together. My husband (6'2") overflows the aisle chair, but they strap him in and cinch him down, and he survives ok. I hover and direct, as, no matter how well-intentioned, people don't understand about spasticity and the unique quirks of each person in a w/c. That is to be expected. The airline employees appreciate our early arrival so they have plenty of time to hunt down the aisle chair, find the extra lifters, etc. We act as though we have done this a million times, and know what we are doing. (!)
We take all loose parts of my husband's manual w/c into the plane with us, and trust the airline will be kind to the chair. They are well-instructed to handle all types of w/c's, but we have never traveled with our power chair. I would by all means take yours - just communicate fully and repeatedly - as for his cushion, that is a crap shoot. Some airlines wink and let you substitute it for the seat cushion, others have a fit. If it is a long flight, I lean harder. If he has an air cushion, be aware that at altitude it will be harder. People don't understand at all about pressure sores, so, educate, educate!
We have always been able to bring a bag (traveling commode stuffed with extra equipment) and claim it as medical supplies, and it goes for free. Sometimes you have to work it a bit, but if you sweetly stand your ground, they come around. Mostly, as soon as you start listing catheters, or commode, they rush you through! :^)
At security, you and your grandson will be taken to the head of the line, and he will go to a special area and be thoroughly searched and patted down. Meanwhile, you will be taking trays of stuff through the xray machines - take as much of his stuff (backpack, hat, wallet, etc) as you can so he does not have to deal with it. We found that they do not like us to come near while they are doing their bit, and I have made the mistake of giving my husband back his hat before they are done - ouch. They have dealt with many people in w/c's, so my husband just relaxes and lets them do their thing, with this special quiet dignity that he has.
Mostly, we find people eager to help, and I would guess your beautiful grandson will win hearts and attract lots of airline employees happy to make his trip fun and successful. Give yourselves lots of extra time, and have a wonderful wonderful time!