K.M.
I'm sure others have recommendations, but how about the Garland Surf n Swim? They have a wonderful kids area and then right across from it is a wave pool area. It is not a huge water park by any means but fun for a couple of hours. It depends on how adventurous your brother likes to be, but that place could work out. Also I've read about a fun water park in Rowlett, also run by the city. Good luck!