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.
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.