I found a small white pill imprinted with an M on one side and L7 on the other side in my sons room

Most likely levothyroxine sodium (generic of Synthroid). A thyroid medication.

The above is for information purposes only and is not intended as medical advice. You should always contact your personal physician with any questions or concerns.

I would suggest checking out information that you might find on the following site.

Pill Identifier - Drugs.com