Just to make sure that I'm understanding your post, you placed a lion fish in a tank with a clown fish and cleaner shrimp? If this is true, I don't think your clown fish will be around too much longer or any other animal small enough to fit into the lion fish's mouth.
As to your other concern, lion fish are normally a lazy type of fish. They like to just hang out and do nothing. Atleast that's how I have always noticed them. So, the fact that your fish is not very active until he's chasing a potential meal sounds about right.
If you are really concerned about his eating something though, this is one test that will tell you alot. If you have some tongs, grab a small piece of shrimp with them and daggle it in front of the lion fish. If it doesn't try to snatch it up, i would be concerned as lion fish will usually take an easy meal.
note: please remove smaller fish from the tank with the lion fish unless you plan on using the smaller fish as food for the lion fish.