E.C.
My 6 yo daughter has had bo after coming from outside. She is almost 7 and it has been for about 6 months. Her daughter thinks though she is going through early Puberty sigh... :( ... as she has all 5 signs of getting her period early (acne/blackheads, bo, growth spurts every 3 months vs 6 mo, getting fat in the breast area & pubic hair)... Said to think it at her age.
Anyways she uses deodarant. I use the All natural Tom's brand and it works great. She puts it on every morning and if playing outside in the evening she will reapply.
Also you can watch kids diet. Is he eating a lot of junk? Horomone based foods?