They are notoriously picky eaters and difficult to get eating in the first place, it's a good thing he's found something he likes at all. Daugherty is on the right track to soak different types of food in garlic extract. You can make the garlic extract yourself, but Kent Marine and SeaChem also make garlic additives for food. Garlic is an appetite stimulant in fish and it may work to get him to try new things. If he doesn't take to anything else, you'll have to continue with the krill.