They can go right up to the top of the waterline, anything he can rest on. That's why I like Water Sprite if you can find it (usually not in big chain stores) because it can be planted or floating and I usually float all of mine and all my fish sleep in their sprigs of water sprite ^_^ it's super cute.
But yeah, any sort of real plants like Valisneria are good for just plain cover, you could have one Amazon Sword in there although that might need a root tab under it to really grow, root tabs are really super easy too, just stick one under the plant and forget about it for about 2-3 months
And other plants you could use would be most stems like Anacharis, Ludwigia, Rotala, some Java Ferns get big too and those will grow in time so that he can swim through them and rest on them
But you can also get some silk plants (not plastic please!) as well and kind of stuff it all in to make a jungle, well it doesn't have to be like a real jungle but whatever suits your fancy and you think that he might like.
For now if you want you can lower the water to about half way so that he doesn't have to travel so far from the bottom to the top, this might help with getting him out of the corner. Hides are also good, like decorations with holes and all. Since fish don't have eyelids they like to sleep in darker places so that will help ease his stress too.
Does he normally have lights on that tank and do you turn them off at night for him?