If the ray has not eaten in a few weeks, and its starting to curl, Im sorry to say it prob will not pull thru.
Some say that adding some salt to the tank will help with the healing, but it might be to late with that already since its been so long.. Add heat, lower lighting. With a burn, there is not alot you can do. And if the ray is not wanting to eat due to its in pain, not alot you can do about that. Maybe get a turkey baster, fill with black worms and see if you can squirt the worms out rt in front of the ray and maybe under him so he doesnt have to work so hard to find food by swimming around.. Help him save energy for healing..
Best of luck..
All I can also say is get that heater covered.. go buy a heater cover that will fit or get a piece of PVC pipe and drill alot of holes in it and slide it over the heater..
hope it pulls thru
so, add some salt if that is your pref.
lower the lighting, leave off for awhile
turn up the heat a few deg.
get black worms and try feeding him thru a turkey baster
thats about all you can do