I vote proper bike.
Anyone that's ever ridden a £50 BSO will know just how totally nasty they are. I'd not wish one on my worst enemy, let alone on someone who's starting off cycling. When you're young, 2.5 miles is a long old way and trying to do it on a shit bike will make it a joyless experience that he'll jack in as soon as he can.
Also, although your average BSO will probably be safe when it leaves the shop (I say probably, because I've seen some frigging unbelievable cockups rolling out of places like Halfords - forks on back to front and so on) it'll not be that way for long. I don't have to tell you that having half decent brakes and tyres are crucial for stopping a bit sharpish.
My suggestion is that you scour eBay for a nice old steel MTB from the early nineties. You don't say how tall the lad is, but if he's anything like most kids these days, a small adult frame may do the trick. Such bikes are going to be kitted out with a better quality of brakes and transmission and you can be fairly sure that even if they get abused, they'll keep on working a hell of a load longer than a new BSO.
If he's not big enough for that, then look at a second hand Islabike or something from EBC like Clarion says.
Either that or if street cred is really important, maybe a proper BMX is the way forward? I mean - built like a tank with big tyres and a single speed? That might be just the ticket, too.