Hi.. Thanks for the query.. Metal smelling breath can be due to causes like acid reflux, metal poisoning,side effects of medications, dental decay or gum infections etc.. Sometimes sinus related infections can also be a cause.. I do not think it is related to cancer so please do not worry about it.. You should consult an Oral Physician or an Otorhinolaryngologist and get evaluated and a proper clinical examination should be done.. Once the cause is ruled out it should be treated for resolution of the bad smell.. For now he can do steam inhalation with a few drops of Eucalyptus oil in boiling water.. Chewing flavoured sugarfree chewing gum and sucking cloves or cardamom can help in masking the smell.. Hope your query is resolved.. Regards.