Wish more fathers were like you Sir!
Get your son enjoying the outdoors and fishing and you will not got wrong.
I do not fish in the Dallas area but a lot of anglers who do post in this section. Hopefully a few will reply. I may suggest to start out at community ponds. You can also search Google for area lakes and rivers in your area. Check out their fishing reports. Also State Parks with water accesses are great places to fish with lots of sunfish. Hope you find some spots.