I always treat for internal parasites, they come in with a high percentage of having them. If he is eating dead food, you can mix metroplex/focus in food and feed at least every other day for 2 weeks; along with dosing prazipro in the water column for the recommended dosage. This method would be considered reef safe, and would only be effective if he is eating dead food.
Because I am usually dealing with dwarf and medium lions my preferred method is general cure in the water column, this method is not considered reef safe. General cure contains metroplex and prazipro combined, which would cover a broad spectrum internal parasites and worms, as well as flukes.
Never ever use copper, even if they survive they will likely not live much longer than a year after exposure to copper. Lions have super strong mucas layer and even shed a cuticle layer, which does give them an unusual resistance to ich, velvet and the such. The main issues I have seen with lions is #1 internal parasites/worms and bacterial infections, paraguard has worked for me with mild bacterial infections. More serious bacterial problems although treatable with broad spectrum antibiotics, usually after extended dosages they succumb to likely internal organ failure after a year or so.
Internal parasites is really almost a given, but if you wanted to observe carefully instead, watch carefully for stringy poo. Look around the bottom of the tank carefully. If he goes off of eating, observe very closely for stringy poo. I would have general cure on hand because if you wait and it is determined he has an issue, you need to move fast. By waiting and if he stops eating it also becomes more risky.