Hi John, and welcome to the forum.
Difficult one this, 5 years old is very young. You need something as simple as possible, yet which will give at least a half decent view of objects in the sky. The big problem is, the less you spend, the less satisfactory will be the view. On the other hand, you don't want to spend a lot of money on something which will just gather dust, although I could weep when I think of some of the things I have spent money on for my daughter over the years.
The usual recommendation for a complete beginner is to start with binoculars, but you could not be sure that your son would be able to hold them up to his eyes for very long, or if he was looking at the "right" thing. So you need a telescope of some kind.
There are two basic types of astronomical telescope, refractors and reflectors. Refractors are the "classic" type of telescope most people would think of if asked to describe a telescope, a tube with a lens in one end, and an eyepiece for magnifying the image at the other end. A reflector consists of a tube with a mirror at one end, and the eyepiece at the other, again to magnify the image. It is generally true to say that a 2" lens is equivalent to a 4" mirror, a 3" lens to a 6" mirror, 4" to 8" etc etc.
There is a wide choice available in both types of telescope, and prices vary greatly. But be sure to buy from a recognized dealer, do not buy from a toy shop, and do not buy from ebay or the like, until you know what you are doing.
Here are just a very few possibilities, maybe some of the other guys will have other suggestions.
Simple refractors
www.firstlightoptics.com/products.php?cat=51
The Skywatcher Startravel 80 (second from the top) would be my choice here. Small, easy to handle, supplied with two eyepieces, plus a tripod, and reasonably priced.
Simple reflectors
www.firstlightoptics.com/products.php?cat=28
The Skyhawk 1145P is a superb little scope, and the Explorer 130 is also good. The Explorer is probably the best selling beginners scope in the UK. You may even want to consider the little telescope at the top of the page, it will at least give a nice view of the moon and will also show the four moons of Jupiter.
Simple reflectors and refractors
www.celestron.uk.com/viewsubcategories.p...eries_firstscopes/36
This page contains more examples of both simple refractors and reflectors. Click on each for more details.
Just one thing, whatever you buy. Make sure your finder telescope is set properly. You can do this in daylight by aligning on a distant chimney pot or tree. There will be full instructions provided on how to do this It will make finding objects in the sky much easier.
Whatever you decide, good luck.
  