How old is your son? and do you expect him to get on with it by himself, or is this something you're hoping you can do together?
£150 is not going to get you a Hubble Space Telescope, but you can get a decent scope that will show you details of the moon, and some of the features of the brighter planets - especially Jupiter and Saturn.
However, although the scope should be reasonable, it's often the case that they are mounted on cheap tripods that give a lot of wobble and may be some effort to use. They can be used to good effect, so long as you're patient and learn to look through the eyepiece without having to hold on to the telescope or mount
In your position, there are two that I would consider;
a 130MM Newtonian Reflector like thishttp://www.tringastro.co.uk/sky-watcher ... 3145-p.asp
This is about the biggest size you're likely to get for your money - but the downside is that the mount can be fiddly to get used to and needs some care when setting it up with aligning it to North etc. If you're planning on working with your son on this hobby, you may be able to help him avoid frustration.
My alternative suggestion is a smaller aperture Refractor like thishttp://www.tringastro.co.uk/celestron-a ... 5835-p.asp
The mount is (in principle) simpler, as it's more like a photographers tripod. The 90 degrees angle at the eyepiece means that this one can be used for terrestrial use too - so birdwatching or scanning the horizon for pirates are possibilities (depending on the age of your son).
Both of these are a bit less than your target budget, and I'd suggest you also get a planisphere which is a plastic device that will help you learn your way around the night sky.
NB - other vendors exist, I've only posted links to Tring, because I've bought from them before. You can also try Green Witch, Harrison Telescopes, Warehouse Express, and there are others too. Other places sell telescopes, but you may not get the same quality of service.