just my 2 cents:
REGARDLESS of stocking he should be moved, at least temporarily, for a few reasons.
#1-while that injury looks unsightly i see no sign at present of any infection/fungus. If you can keep it that way he will heal up fine. You need to keep the water PRISTINE and also cut down on ANYTHING that might be a stressor to the fish which will hamper his immune system. As it is he seems to be doing a fine job of fighting off any nasties that come with an open wound like that.
#2. as mentioned above, fish will often pick on weak or sick tankmates...asking for trouble (see #1)
#3. If he is isolated you can treat appropriate with aquarium salt to further assist in healing. Melafix is useless IMHO. Salt cannot be added to the tank as you have a plec and RTBS which are both salt-sensitive. A small floating container as mentioned in previous post is a good idea. Do 2x daily waterchanges!
You asked for advice to help the fish. Take it or leave it. There is no reason to euthanize at this point but you have to do what is right for the fish right now so it doesnt escalate to that point. Honestly it could go either way with a wound like that, but right now he seems to be in the clear...just needs appropriate care to heal properly.
all the best
cheers