You won’t be alone, but you won’t be in the majority. I think there are more adults without kids than single parent/child combos.
If your son enjoys the kids club, be prepared to discover the world of solo cruising. For an introvert, just read a book or don’t look available to chitchat. For an extrovert, enjoy the interaction with other extroverts. Even couples who are cruising without kids - or whose kids are playing in the clubs, will talk to solo travelers.
Some kids love the clubs, others want nothing to do with it. You may not know what your son will do until on the ship.
A first cruise of 3-4 days is just a learning experience. Don’t try to ”do it all.” You will have a daily list of activities. Find some that look interesting to you and figure out how your day is going. Adjust as needed.
You will be assigned a dinner table and time. The actual restaurant will change each night, but your waiter goes along. Everyone has the same set of menus, so you don’t miss out. Your son (and you) can order off adult and/or kid menus.
Breakfast and lunch are available in the buffet (Marceline Market) or one of the dining rooms. These have set and limited hours. Fast food is available on the pool deck. This is from 10:45 - 6 pm, with pizza available until before midnight. A limited menu Room Service is free 24 hours.
Soft drinks, coffee, tea are available, self-serve, 24 hours, on the pool deck. These plus milk come with meals. Juice at breakfast.
Castaway Cay - Disney’s private island - is a fairly peaceful day at the beach. Lunch will be available for a few hours midday, including drink station and soft-serve ice cream (basically just the ship’s offerings on the island). Loungers and umbrellas provided, as are towels and swim vests (also in the ship). The Famiky Beach is on a protected area - with a breakwater wall keeping you from the open sea. There is a barrier net to keep out larger fish. Lifeguards on duty.
Shows at night are family-friendly in content and length and are well done. Generally, you should not miss the entertainment due to lack of seating (as is a concern on other cruise lines).
That’s some of my thoughts. If you have more specific questions, ask away, and get ideas from others. With a new adventure, it might be a challenge to know what to ask. But cruising (especially Disney) does not require a lot of planning.
Make sure you have a state-issued Birth Certificate or passport for each of you.