I believe modern thinking is lateral line errosion is caused by nutritional deficiencies (someone correct me if I'm wrong). Some still attribute it to other unvalidated factors such as stray electricity or simply the use of activated carbon. A grounding probe wouldn't hurt if you're not using one.
i've had lots of tangs over the years and never had one develop lateral line disease (although i've nursed a few back to health from this problem). best thing to do is a varied, healthy diet as mentioned... lots of algae sheets, formula I and II, mysis shrimp, etc. feed him a healthy diet and watch him recover
Also try improving your water quality with some water changes. I´ve seen a blue tang with lateral line erosion recover in just a few weeks after some heavy water chagnes.
Marco