You wouldnt want sand or rock in there anyways since most meds will kill the bacteria. I would get an AquaClear HOB
to provide circulation and as a way to add filter media (carbon) for when you need to clear the water of the meds they are not the cheapest, but seem to be the easiest to use and have a decent all around build quality. They cost a a few dollars more and are worth it! I went the cheaper route and wound up replacing TWO different HOB
filters before getting the AquaClear...Add a decent heater and thats all you need. No need for a hood or light or anything except maybe something to keep a jumper from carpet surfing.
Also, add something as a hiding place. A CLEAN PVC
coupling should do nicely...
See, very cheap and basic!
I would make sure your son cannot reach the tank as I have read too mnay stories of little ones trying to help out when they learn of a sick fish or a tank in need.
Good luck with your fish! Sounds like he needs some Pimafix or Melafix or whatnot along with good water quality. Youll want to check the tank water for ammonia daily and do PWC
to bring it down.