My son is 6 and has multiple osteochondromas on his bones, is this common in children?

1:50,000 is the : overall incidence. Multiple Hereditary Exostoses, Types I, II & III map to genes on chromosomes 8, 11 & 19, respectively. Multiple osteochondromas can occur in a few other syndromes, also. See http://ghr.nlm.nih.gov/condition/hereditary-multiple-exostoses. His pediatrician or pediatric orthopedist can order a Chromosomal Microarray & refer him for a clinical genetics evaluation.