Hello, sorry for the delay in replying.
Body odor is typically a result of androgen hormone action. Typically, before puberty androgen levels are low in both boys and girls. Thus, body odor in young children should warrent evaluation.
The presence of body odor may indicate an increased androgen level. Other signs that this may be occuring are accelerated growth (I.e. your son seems to be taller than most kids his age.) and axillary or pubic hair.
I would suggest that you discuss this with your pediatrician. There are some blood tests that may be useful. These would include an electrolyte panel to look for abnormalities that may be indicative of a partial congenital adrenal hyperplasia (CAH), TSH/T4 to test the thyroid, and various tests to look at androgen levels directly.
If all these end up normal, it is likely that all is well; however I would suggest continuing to follow with your doctor and monitoring for any accelerations in growth, etc. that may occur in the future. Basically an annual well child check.
Hope this helps.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.