These answers are for informational purposes and do not replace a physician head-to-head visit. A patient-physician relationship is not established.
I hope I can help answer your question.
From your description this sounds like a common entity called Juvenile Palmar-plantar dermatosis; sometimes referred to as the sweaty sock syndrome. Usually this does not have symptoms. Usually affects only the plantar surface of the feet. Some authorities feel it is do to repeated moisture and drying. The foot is in the shoes and under socks, sweats and then when the shoes and socks are taken the skin immediately dries and can crack...much like ground after the sun comes out.
It is best to treat this by breaking this wet to dry cycle.
Your son should change his socks as soon as he gets home, and change them frequently. He should use white cotton, socks. At bedtime, he should apply a thick ointment such as Vasoline or Vaniply ( I am not sure if this is available in the UK).
I hope that helps and please let me know if there are further questions.