raleigh is always a really good bet. The cheaper ones are often poorly made and can be much harder to ride (especially for a young child). If he is average size for his age I'd have thought you're looking at 14 or 16 inch (wheel size). IMO it is worth taking him into a shop so you can get the right size - again this is really important when they are little. My DS1 (4.5) is on a much smaller bike than his friends because he has very little legs. You can work it outby inside leg measure (see on raleigh website), but it is better to try him on an actual bike. That said it is cheaper to buy online.
We just got ds (5) a Ridgeback bike. dh uses one (bigger, obviously) for his work commute and they are very good well made bikes. Not cheap, but then even the crappiest bikes from Toys R Us are expensive too. I think his retails at £130 but we got it off ebay for £30.