If Lancelot came from a shelter you probably don't know what his home life was like before. While I agree that cats play well into their senior years I find they get out of the habit if no one encourages them by playing with them. If you engage him more yourself he might rediscover the joys of play. But he'll still probably enjoy it most if you are on the other end of that piece of string.
All of my cats love a slender stick moved about under a scrap of carpet. I poke the stick out one side and they pounce. I move it out to the opposite side and they pounce. I move it under and they pounce on the moving ridge under the carpet. Often they dive under and try to catch it there and end up wiggling along under the carpet to the other end. Or they forget the stick and wrestle with the carpet. Takes me to get them going though, usually.
Did you notice the warning about the string hanging from the doorway?