My son has be racing road bikes since he was 9. He a skinny kid as well. I had a hard time finding cycling clothes for him when he was young. The best place I've found for quality and selection is Rogelli. They will have what you need.
I am a new member here, so I can't post web links yet, so change the (dot) below to a period.
www(dot)rogelli.com/en/cycling/kidscycling