Common cold, as the name suggests, is quite common, specially in young children. On an average, a normal child may have 5-6 separate episodes of common cold during a year. Each lasting 6-7 days and sometimes more. What you are observing in his nasal discharge are probably mucous flecks which can resemble food particles. Also, specially during sneezing, as respiratory secretions forcefully come out, any food particles stuck in the pharynx are also likely to come out with it. That is no cause for concern and is quite normal.