San Antonio has Sea World and Fiesta Texas. Remember, by March it'll be pretty warm and just like other amusement parks, there's alot of walking. I suggest getting your tickets online in advance, and getting there when the gates open.
If your son like walking and looking at things, then he'll like the Riverwalk, the Mission Trail, and the botanical gardens to run around in. Hemisphere Park is right off the Riverwalk next to the convention center (which also has an amazing underground landscaped area the entire length of the center) that has fountains, misting stations, play areas and cafes.
There is a great children's museum a few blocks from the Riverwalk - interactive and great to introduce him to Texas-Mexian culture.
The Alamo isn't crowded first thing in the morning. We go all the time and have great pictures of us in front of it w/o anyone else in the picture. Its in Alamo square where you'll find plenty of refreshments, my favorite being the Hagan Daz shop right across the street.
Don't do the bus tour - waste of time and waaaay to expensive.
Texas has alot of history - western and Mexican. We have stayed at the Menger, Emily Morgan and Crocket Hotel - all nice and old w/lots of historic pictures in the lobbies and hallways ifyou like things like that. If you don't stay there, at least go through their lobbies - periodic furnishings and marks of history everywhere. We have also stayed at several hotels right off the riverwalk - not bad as long as you have a riverview.
Mi Tiera is a great market and has a great cultural atmosphere to eat and shop. Try some of the Mexican pastries! You can hop on the streetcar line to get there. We tried walking once and in the heat it was too much for us, but it really isn't far from the riverwalk on a nice day.
I suggest visiting Buckhorn Saloon - its a restaurant and museum of the most mounted wild game I've ever seen in my life. Chris Madrid's is worth a visit - they have the best and biggest hamburgers ever, but its out of the city limits. But anywhere you go is good eating in San Antonio.
gl! We love San Antonio!