I'm completely in agreement with other moms: NO TV! It does unnatural and unrelaxing things to brainwave patterns. I always notice I feel pretty awful when I watch TV, and haven't had one of my own for many years.
Aquariums are wonderful soothers, but require care, and tapping or clonking things against the glass can jolt the delicate nervous systems of fish. They have nerves along the lengths of their bodies that pick up vibrations with great sensitivity. (Imagine gunshots going off in your bedroom…!!!). Not to mention the possibility of breakage when little boy games get lively. And little kids have a yen to put the wrong sorts of things in with the fish, sometimes deadly for the little finny guys.
My grandboy has a not-too-bright "aquarium" light with fish that swim in an endless loop. It gives some of the same monotonous entrancement without the risk or maintenance issues, and is not expensive.
If you get him one of these, you might make a big deal about it being a reward for staying in bed when you present it to him. Tell him a detailed story about how much a little boy just like him enjoys having his little fishy friends for nighttime company, and how they help him relax and go back to sleep. Have him practice turning the lamp on and off. Ask him if he'll stay in bed if he gets to have this light. He'll probably say yes, and then call for you anyway at first. But keep reminding him, and he may catch on pretty quickly.
My grandboy also responded well to reward charts at that age. Keep the initial rewards almost immediate, and gradually add nights as he improves.
Good luck. This is a challenging problem, and some little kids are just not very good sleepers. As a person with chemical sensitivities that take a serious bite out of my ability to sleep, I often wonder whether our kids are just getting too many chemicals in the air they breathe: cleaning products, air "fresheners," fabric softeners, mommy's perfume, off-gassing from synthetic products, etc. If you have any reason to wonder about this, you might try limiting his exposures for a few days and see whether it makes a difference.