It depends on his BODYFAT composition. If he's all muscle, what you're proposing is that he basically starve himself so that his body uses muscle breakdown to provide energy for daily activities? That's not going to happen.
12- 13% is fine. What, is he going for professional status? Will he ride for a professional team in Europe? What's the goal?
Seriously, you have to be realistic when setting a target goal in terms of weight and cardiovascular for the individual. It's good he's going to race, and there's not a problem with where his bodyfat percentage is for the level of racing he plans on doing, but I wouldn't recommend any further loss of fat. Normally, a male athlete will have about 5- 12% bodyfat, and he is well within the acceptable range for bodyfat for a male. But just because he's on the high end of bodyfat recommendations for males, doesn't mean he has to starve himself down to 5%. He is fine, and I don't see any reason for him to starve himself down to unrealistic levels.
Simply looking at numbers and making loose translations has no benefit to anyone in training. There is more to the picture for someone than deducing that because you're 5' 10" and 188 that he's overweight.
Koffee