Make an appointment to see the headteacher. Take a friend or relative with you, (just in case there is any 'confusion' about what was said at the time).
Explain to the head that, whilst you pleased that you son is thought to try hard, this is obviously not enough and that you would like a written plan of what the school intends to do to get his reading/spelling levels up to standard i.e. for his age of 14, and, staying calm, that you will be looking forward to speaking to those members of staff who will be responsible for his future and immediate development. Mention that you are more than happy to work with your son in the evenings and weekends on any extra work that his teachers set him.