siphon the water out of the overflow until there's only about 1" left, then net him.
If it is doing fine then you should just leave it there until you have another reason to tear down the drain and return pipes. I had a 2" front disappear for almost a year. I found him in a small compartment in the filter at 4". He wasn't super healthy when I found him but now you couldn't tell the difference from the others in his colony.
I suggest try using a plain net or plastic bag attached to a string to fish him out.
If it is doing fine then you should just leave it there .
I'm with this idea just leave him in there. He eats the food that goes through the overflow so he's doing some good.