We have done many cruises and WDW trips, and my son definitely does better on the land than on the seas. Our first 7 night cruise with a 4 yo with special needs felt like we were trapped in a luxurious prison cell for a month. I swear that was the longest week of our lives. Definitely start him out on a shorter cruise! At 8 yo our son loved the Disney kids club, but still wouldn't stay for more than 30 min by himself. Fortunately, Disney is the only one that allows all ages of kids together, so he was thrilled to hang out as long as his sister was there too. Plan on keeping him with you and visiting during open family time.
Every child is different, and our son actually loves the characters, rides, fireworks, and total chaos at the parks. He also prefers the extra space and private pool when we stay off-site. On cruises, he enjoys the live music and pools as well as just being with us 24/7 (no work, school, therapy appointments, etc.) The rest of us love cruising enough that we keep trying it with him and now he does a lot better, but he would probably rather be home with grandma given the choice. Unfortunately, he is a bit too much for grandparents to handle for extended periods. You will figure out what works best for your family.