Girls, share your experience. Baby is 18 days old, fully on breastfeeding…

Girls, share your experience. Baby is 18 days old, fully on breastfeeding, yesterday weighed him, weight almost didn't gain, 250 grams in 18 days, sleeps 2.5 hours both day and night. Breastfeed on demand, but weight gained just a little bit. Should we switch to mixed feeding? After breastfeed supplement? Or daytime with supplement, and at night breast? Who manages this situation in our case?