6 yr old with armpit odour

my 6 year old son has started to get this "onion like" odour under his arms when he comes home from school. He takes a bath every night and has a wash in the morning before school so hygeine is not the issue ( I scrub his pits myself ).

When I mentioned this to the family doctor she just said to make sure he washes well at bathtime...which we already do. She does not seem to think that there is a problem.

I do not want to use deoderant on him because I think he is too young for that.. any suggestions??