supplementing with breastmilk?

Jasmine • Mother of 2! #momofboys Instagram: jasroc3
I am exclusively pumping my 7w old son. He NEVER whether it's during the day or night sleeps longer than 30-45 mins at a time. I'm curious if I should try supplementing with a formula maybe before bedtime to see if that helps him sleep longer throughout the night? Has anybody tried this, or maybe have some other tips? 
Thanks!!