I have a 10 yr old and she has a Nintendo DS (stands for double screen). It will play gameboy advance games and DS games and is 149.99 in Argos with the Nintendogs game (all about raising and training puppies) page 1589 in the new catalogue - might be too girlie for him. Game boy advance is on page 1591 and is cheaper. Depends how much you think Junior's going to play with it. PS2 is same price as DS and played on yr telly - but geared towards an older child - majority of the games are tactical, sport or war related. If you're unsure I'd go for the Gameboy advance - the games are cheaper and some have 2 games on the same cartridge - he can always get DS or PS2 later - xmas, birthday, confirmation.
I'd say Argos, Smyths or World of Wonder are your best bet for prices - especially heading upto Xmas. They tend to do bundles around that time - 1 or 2 games and the console for X amount, and vouchers for future purchases. Most of the games are for age 3 and over. So well within the remit of your average 6 yr old.
Why not take him with you to Smyths/World of Wonder for a mooch and see what he interests him to get a better idea. I tend to leave the catalogues kicking around so Poppy can have names of things for her Santa list.