I would say that if you already have a desktop, then get a laptop. You can then both share the 2 computers according to what you want to do.
As I spend (too many) hours on PCs, it can get a bit lonely and "cabin feverish" from being stuck in the computer room/office all the time.
It's nice to be able to sit anywhere in the house & garden with the family. So in this respect the mobility, space saving and "real human interaction" of a laptop are very desirable.
You could even take it anywhere in the UK with a mobile internet dongle from around £10-20 a month, or connect to many public networks available.
When I want to play games I'll go to my Desktop Gaming PC.
If your son wants to play recent graphically challenging games such as Call Of Duty 4 for example, then I would get a desktop.
Even so-called gaming laptops costing thousands are generally pretty rubbish for games, so you can imagine how disappointing a £430 laptop would be. That said, if your son only wants to play simpler games such as Sims 3 then I would think that the GeForce® 9300M GS would be ok.
One last thought, laptops aren't as easy to repair as desktops. You can replace/ upgrade every part of a desktop cheaply and easily yourself. Motherboard, processor and graphics replacement on a laptop are more expensive and difficult (or not possible), even if you can get hold of the parts.