Your baby shouldn’t have body odour at this age, you should see a Paediatric Endocrinologist

Sweat is a major cause of body odour especially in the armpit.

It's part of growing up. Adrenarche which is part of puberty is occurring here. Never mind. Just a regular bathe and deodorant for kids is ok.
Some children have early Puberty which can make them have body odour...Just focus on hygiene. If you are worried, kindly see a Paediatric Endocrinologist for further evaluation and management