Late Developer

My 14 year old son is quite small for his age and not very sporty. This didn’t seem to matter in primary school but the other boys have started to mock him and push him around a bit and I saw nasty remarks written on his schoolbag recently and I don’t want this to go any further. What should I do and who should I talk to? He does not want me to approach the school but I am at my wits end?