At the risk of being blunt, you haven't gotten any good advice thus far. You really don't want any loose substrate with these guys whatsoever... anything that small and not tied down in their enclosure is liable to get swallowed, and swallowing a big piece of moss or gravel could lead to their death.
They also create a massive amount of waste, which can become very difficult to clean up as it soaks into the substrate... especially because they need a very wet environment. The smell of dirty, wet toad substrate is something to behold. So first things first... ditch the gravel, and definitely don't put any moss in there.
I set up my firebelly toads with a water substrate, and then built them an island to haul out and bask on. This enclosure has worked well for several years now. Being semi-aquatic, but also semi-terrestrial, it is imperative that they are able to swim and move around on dry land.
Like all amphibians, they require a varied diet to thrive. These guys are big eaters and will pretty much eat anything that moves. I feed mine crickets, mealworms, waxworms, chopped nightcrawlers, butterworms, and frozen bloodworms from tongs. As they get more used to you bringing them food, they will come to the front of the cage when they see you, which makes tong feeding easy. Incidentally, this same behavior is what makes not having loose substrate imperative... when they become frenzied, they attack everything in sight, including cage decor, fingers, and each other. They don't have teeth, so they cant hurt us and the usually don't injure each other, but they can and will end up eating the substrate that way.
Their food should occasionally be dusted with a calcium/vitamin D3 supplement. They are not choosy about water temperature, and will thrive if temperatures are anywhere between the upper fifties and the high seventies.
Hope your son enjoys his toads, and feels better soon.
For reference, here's my enclosure, and my toads. I've found this type of enclosure to be by far the easiest to clean, safest, and most dynamic way to keep semi-aquatic amphibians.
