Try regular table shrimp. Buy the raw with the shell ON. Slice a piece (retaining the shell) a little larger than the krill he ate (be sure to thaw to tank temperature - maybe 5 minutes in tank water). If he takes this, then it can be a primary part of his diet but you will want to alter with other foods, preferably twice a week. We typically alter between blue crab claw and shrimp with occasional, live clam, live fiddler or crawfish (depending on the season either just the frozen tail or a live animal - must be watched as they don't live long in SW but they are usually captured and eaten before showing obvious stress).