They bounce well though 🙂 . Take him to a shop and try them on – they all feel different and are cut different. We got ours from Urban Air and tried on about 4 differnt kids elbow and knee pads and gloves.
Definately go along to Pirates as they'll help you size a bike if you do take the plunge. At 5 he'll probably be on a "micro mini" or a "mini". The downside is kids race bikes don't come cheap – 250 and up for a new one. Second hand they get snapped up quickly as well.
If you want to do some surfing you could have a look on BMXtalk which is a bit more race orientated and gear comes up for sale fairly regularly.
Shops to have a look at to get an idea: Groove Racing, Tibbs, Alan's BMX, urban air. For a 5 yr old, the lighter the better. Redlines usually win here but they don't come cheap.
Have fun!