I would get a minimum of 3 ducks. This way if an accident happens they are unlikely to be totally alone. They do not cope with being alone, and will cry out continually until they are reunited with their friends. When our Cyprus broke his hip we had to reduce his mobility via a kennel for 6 weeks. We were unable to leave him in our house during this time. He cried and cried for his brothers. and his brothers stood at the window crying and crying for him. So we moved him back to the duck area in his kennel and they were able to chat to each other. His brothers would lay down next to the kennel to rest. Everyone was pleased when he was finally healed up. Ducks are very gregarious and not really territorial, and even wild ducks may easily join up and socialise with domestic ducks. Ducks are very happy to be with ducks!
I would also say they are nothing likely chickens in terms of care, other than living outside.
Do you know what types of duck your son is interested in?
It may help me with more detailed questions. (Especially in terms of duckling diet.)
For your ducks' house try to get one on ground or very close to the ground with a minimal ramp, they don't usually like heights. We put them to bed in their house to prevent injuries from night panics and attacks from predators... But our boys never put themselves away like the chickens. They are trained and know we will be putting them in so when it gets dark they collect near the door, but we always need to encourage them to actually go in.
There is a duck pellet that we offer in a pan, and we also give them left over veggies, peas and corn. They love greens. Always place their feed very close to their water. Ducks must use water to swallow their food. Dry feed can build up in their throats and they can choke without water. If you have enough area the ducks can also forage for most of their own food. You would be amazed. We have them on about 700sqm and they sometimes don't even eat the pellet at all and find their own stuff.
For water you should have a large waterer in addition to their pond/pool. We don't have a natural pond. (When we buy a house soon, we will hire people to dig one properly.) We use a child's pool that we bought at the BigW. We did a bit of trial and error before we found the right child's pool. Cyprus broke his hip from leaping out of a hard shell pool. So we decided to find something better. We one that has a rigid but flexible edge now and they can enter and exit without flopping out. We have been using this style for more than a year. And we did need to replace it after about a year. They are in their second now. It is also easier to drain with the edge.