Your son can apply for his licence up to 3 months before his 17th birthday, but it only becomes valid ON his 17th birthday.

So the earliest he could take his theory test is his 17th birthday. However, he can start studying for his theory test as soon as he likes.

He can also start driving lessons under 17 on private roads. We offer practical driving lessons from age 15 on our private road system.

These lessons follow the DSA syllabus and are given in dual controlled cars, by approved driving instructors (ADIs) fully insured for off road training. They are also supported by lesson handouts to review, reinforce and revise the lesson content.

Having practical lessons at age 16 would help your son with understanding the theory as well as allowing him to gain confidence an competence in car control and procedures before venturing out onto public roads for the first time.
More information: http://DriveAt15.com