Energy is not a tangible thing, it isn't something you can touch, see, hear, taste or feel, energy is a concept, just like a number is a concept, just like speed is a concept. It's not a real thing like a tree, a bacteria, water, or the Sun.
And because energy is a concept, it is misleading and a source of confusion to use language such as "waste energy", or to say that some object "has energy", because how can you waste something that is not real, and how can an object possess something that isn't real?
So don't tell your son that wasting hot water is a waste of energy. Teach him instead what it takes for him to be able to have hot water for his shower. You may have to do some reading up on your part, but then you will both learn something!
Tell him that water is transported to your home through underground pipes, and when it reaches home and your son turns the hot water knob, this water gets heated, usually with gas or with electricity. (If he is interested this could be a good opportunity to explain to him how gas and electricity can heat water: gas is ignited and burns; electricity is a bunch of tiny particles moving inside a material which make the material get hot, and when this material is near water it heats the water).
If your water heater runs on natural gas, tell him that gas is extracted from deep beneath the ground, which takes a lot of effort, and that there is not an unlimited supply of gas below the ground: the gas that is burnt cannot be used again.
If your water heater runs on electricity, tell him that in order to generate electricity (i.e. to get the bunch of tiny particles moving) people usually burn coal or burn gas. Just like gas, coal comes in limited supply (and burning coal isn't nice for the environment: http://en.wikipedia.org/wiki/Coal#Environmental_effects). There are also the radioactive nuclear fuels which come in limited supply as well, but it would be difficult to explain to a 5 yo how those are used.
Bottom line is it takes a lot of effort from many people to gather natural resources which in the end allow your son to have hot water, these resources are in limited supply, and the more he wastes hot water the more he uses up these limited resources. So by not wasting hot water he thinks about others and not just about himself.
Now if you want to explain to your son the physical concept of energy this isn't straightforward. How would you explain it to a 30 yo?
Personally I think we would be better off without the concept of energy at all. The concept doesn't explain anything, worse than that it hides real explanations and gives people the illusion they understood something while in fact they haven't understood anything at all.
To give an example, often people say that a ball thrown upwards decelerates on the way up 'because' its kinetic energy is converted into potential energy, and then accelerates on the way down 'because' its potential energy is converted into kinetic energy. But an abstract concept cannot be the cause of anything! This is called the fallacy of reification, the logical fallacy of treating a concept as a real thing, as a cause.
There is no real thing inside the ball called kinetic energy that gets converted into another thing called potential energy. The ball decelerates on the way up because it accelerates towards the Earth, and it accelerates on the way down because it keeps accelerating towards the Earth. Then we can wonder why the ball accelerates towards the Earth, but the concept of energy will never answer "why" to anything, it can only serve to describe "how". Kinetic and potential energies are just numbers that we can use as intermediaries in calculations. In fact we could reformulate the whole of science without the concept of energy and still describe the universe as accurately.
To give another example (that the concept of energy is a source of confusion and doesn't explain anything), you didn't know how to explain to your son what the concept of energy is, your son didn't understand what you wanted to tell him, and the concept prevented you from realizing that all you had to tell him is that the Earth has limited resources and wasting hot water is wasting these resources.
So if I had to explain what energy is, I would say that it is a dispensable mathematical tool widely used in science and which is a source of much confusion.