He is getting the yeast from your breatsmilk most likely, since he is so young and not on solids yet.
Unfortuately yuowon’t be able to clear his yeast until you clear yours as you will always be passing it on. Read this blog post for tips for yourself: https://holisticspring.org/2023/03/29/10-ways-to-improve-oral-thrush/
For him, coconut oil on his gums or on the breats can be helpful and then giving him probiotics (if your doctor says its ok) can be helpful. you can put it on your finger then in his mouth or rub it on the brest or mix ina bottle or in a syringe.