M.L.
San Antonio is great! We go every year just about for at least a weekend trip. Sometimes up to a few times a year. Sea World, and, believe it or not, Fiesta Texas. Fiesta is awesome, and, my kids love it. My daughter could ride almost every ride there when she was 2 1/2. It is VERY family friendly, and, just tons of fun. Also, downtown is just a lot of fun. I suggest a small wagon or a stroller appropriate for his size. His little feet will get tired from all that walking! :)
As far as hotels go, we usually stay somewhere within the middle of the Riverwalk and Sea World or Fiesta TX. It is a short drive, and, you can find cheap parking lots in downtown. So worth it to save the money and take a 10 minute drive in to where you want to go. We keep saying we want to stay on the Riverwalk one day, but, it is pretty high. There is a La Quinta right in downtown, but, not on the riverwalk that is pretty reasonable. We have stayed there once before.
Have Fun!
http://www.visitsanantonio.com/index.aspx