You might want to keep them separate for awhile and then reintroduce them, slowly, with rewards for good behavior on the part of the dog...or back outside again if he misbehaves. Getting him fixed might be an option. Getting older should help eventually.
Getting banished from the pack for an hour or so every time he makes a mistake....that might help a lot.
But I have heard that keeping two waring animals in totally separte rooms for a few weeks and then slowly, under strict supervision, reintroducing them, is a known therapy.