what is the cause for this condition and your plan of management?

a 14 yr old boy came with chief complaints of deformity of rt forearm
h/o trauma 6 years ago..was hit by teacher at school
later he was taken to an outside hospital where the reports suggest he had been operated for compartment syndrome and the operative notes say that they found pockets of pus

o/e
valgus deformity at forearm
midline healed surgical incision
movements are normal except ulnar deviation at wrist

x-ray