Your first move must be to take the cat to the vet to eliminate any medical
reason why he is avoiding the litterbox, e.g. urinary/bladder crystals,
urinary tract infection, etc.  These make urination very painful, and the
cat is associating the litterbox with pain.  If this is the case, he is
peeing in the living room to let you know there is a problem.  Make sure
the vet does a full workup to eliminate any health reasons for the
inappropriate elimination.  If this is the case, a course or two of
antibiotics will clear it up.

Dr. Elsey's Cat Attract litter (www.preciouscat.com) is made with a special
mixture of herbs that attracts cats to the litterbox.  You might want to
try this after any medical problems have been eliminated.

Nature's Miracle and Anti-Icky-Poo are two cleaners I have used that work
very well to eliminate pet stains.  You can basically use anything that
says it contains active enzymes in it.