left nipple inverted with lump

Hi, welcome to the ehealth forum and I am glad to help you.
You seem concerned with the lump under the right nipple of your baby. It is common for babies, no matter a boy or a girl. They are caused by the hormones that you have that are passed to your baby in the final stages of labor. As the hormone levels return to normal, the breasts will reduce in size. The lump will go by itself. Don't be tempted to squeeze the breasts. Just leave them alone. Mostly the lumps disappear in a few weeks or may even take months. Babies who are breast fed may have breasts for longer. Occasionally, a small lump will persist. As long as the skin over the lump looks normal there is nothing to worry about. The same hormones that cause the breast development may also cause a discharge that can look like milk. It is called "witches milk". Don't worry, this is normal for some babies.
A lump in baby breast is usually quite benign and is not cancer or anything else to worry about. Cancer of the breast is very rare in children.
You must consult a pediatrician if the skin over the lump becomes red or warm and for definitive diagnosis as the same cannot made online.
Hope this helps. Take care.
Note: This post is not to emphasise final diagnosis as the same cannot be made online and is aimed just to provide medical information and no treatment suggested above be taken without face to face consultation with health care professional.