He will need physio but as he's only 10 he'll heal quickly and won't need much - children heal very quickly. Because the plaster will be immobilising joints, the muscles will loose strength and the joints will become stiff. He will need appropriate exercises to get him back to 100 %.
As for it being out by 5 % - what does that mean? does he mean it's going to be shorter? or does he mean it's changed the angle of the tibia? - likely he is referring to the size of the tibia - this is common as the other side will continue to grow while the fracture side won't until the fracture reunites... if you think about the size of a 10 year olds tibia and take 5 % of that, it's not going to amount to much. Most people have some level of leg length discrepancy and its generally asympomatic. if he gets back pain in the future or something related to the leg length difference then he can get a larger heel put on his shoe etc to correct it. my father broke his tib when he was 4 and has had a shorter leg his whole life but never had any problems from it.
I think you can be pretty happy for now... there is potential for the injury to be problematic but make sure he sees a physio and they can guide you through it.