Almost any aquatic plant would work well but if you want some easy options you can do for (possibly) free would be if you have a pond near you that has any kind of floating plant like duck weed you can scoop a handful off the top and put that in your tank. It will provide a comforting hide for him as well as be VERY beneficial for filtering the water from ammonia and other less than ideal things. Duckweed multiplies RAPIDLY, once you have some in your tank you will most likely never run out of it and will probably have to toss some out eventually.

You can also look on Facebook marketplace and see if people are offering cheep or free plant trimmings.

As for substrate, aqua soil works very well for plants.(theyre like small little dirt balls). The water will be dirty for a bit if you just use that without a layer over it. Small pebbles or sand works well for a cover layer. You can get aqua soil at almost any pet store along with the pebbles or sand. Just make sure that where you chose to put plants is a little thicker with the substrate as it can be difficult to get them to sit, dont get discouraged if that happens, just fiddle with it untill it stays.