I got one for Christmas & I have to say as a twenty-something I'm still having problems getting to grips with it.
I think for a young child it's very unsuitable as it has a very small screen & I think that prolonged periods of use without break would put massive strain on a child's eyes. Also, how do you monitor his gaming time & what do you feel is appropriate?
Argos do an ingenius PS2 game lock which fits over the PS2 (new edition) and you can set the length of game time etc. As a parent if I HAD to purchase a games console for such a young child I would get something like the PS2 & invest in the timer lock.
We have a 6YO in our house and got her a V-Tech V-Smile last year for Christmas, it's similar to a games console but much more chunky, less easy to break & only plays educational games. I would consider this a much better buy for a boy of 5 if he has his heart set on a games console. V-Tech do a similar unit with it's own small screen which doesn't look unlike a gameboy which might suit.
I can't emphasise enough how important it is to monitor the use of any games console, the type of game & their content and the amount of time spent on a console. We also invested in an egg timer & it is set to an amount of time for a game - if she's bold, the timer goes down & everyday she starts off with the same 30 mins, so if she's horrid, she gets no gaming time!!!!!
HTH