I would recommed some JB Bore Paste. It will remove the rust but of course will not help with the pitting. I would to a real tight patch fit scrub. Clean the rifle first, then put a good quality gun oil down through the bore. After that with a tight fitting patch, a heavy amount of JB Bore Paste. It will work into the pitting that way, and he can scrub the rust all out. Every twenty or thirty strokes, change the patch. Do about 100 strokes. Then clean the rifle bore and remove to oil it back up as you have it down to bare metal.