Is he eating at all, after he eats he needs to have a noticeable bulge in his belly. Some people see them nibble a few pieces and think "great" he's eating, he really needs to pack away bits like mysis, Brine is not a suitable food, unless he is small enough to need live brine to start. I've found the tinyest ones and even start them out on the smallest ghosties I can find, they are more delicate and almost always have to eat something live. Rarely have I see them easily take to eating frozen food and usually don't eat enough when they do. If you are feeding a lion dead food it's best to target feed them size appropriate sized chunky meat. Either by training them to receive the food as you drop it in the tank, or with a feeding stick, they will rarely eat anything that hits the bottom. Feeding is the most difficult part of keeping these guys, feeding live at some point will likely be a necessity. Personally I advocate a live diet and am not in the school of thought to "train your lion".