T.S.
How important is Bob the Builder to him? Is it for a ride on or just something he wears? Must it be a matching set? Would a set like for skateboarding work for him? If he isn't attached to Bob, you can get plain black knee and elbow pad sets at Target in the isle where the sell skateboards. (here's an example)
http://www.target.com/p/Bell-Sports-Tony-Hawk-Skate-Pad-S...
Then you can get whatever helmet you need.
HTH
T.