HI, You milk probably hasn't fully come in yet. He is just producing more milk for himself. Slepping 6 and a half hours is fantastic for 5 days old. I would say he is just building up his milk. If you are still worried I would talk to a lactation consultant.
I wouldn't rush to supplement, unless the baby has jaundice. Have you been to your first well-baby appointment? You could talk this over with the pediatrician. In the meantime, let the baby suck, it stimulates your milk supply like nothing else. If the baby is getting a supplemental feed, it will result in you not producing to your max over time.