I'm big on trusting your instincts. Even if your kitty isn't exhibiting specific symptoms, the title of your thread says it all to me. A kitten that young should not lose its spark. Your instincts are telling you that something is wrong. Get your kitty to a vet and tell him/her exactly what you posted here. Cats are magical when it comes to hiding symptoms until it's too late. Your decision to keep him indoors is a wise one; cats live much longer, healthier lives indoors but if he isn't thriving, you owe it to him and to yourself to have him thoroughly checked out by your vet. Many prayers going out to you. Please keep us posted.