Mine lost that much too (vaginal birth but EBF). After 10 days she had still not put on much but she'd made a nice gain after 14 days and was back at birth weight by 21 days.
The midwife said to me that some are just slow burners and take a little while longer to get going.
She's now nearly 10 weeks and is consistently tracking the 50th centile for height and weight. I haven't topped up with formula.
The advice I was given at the time was wake her for feeds if she isn't feeding every 2 hours. That was easier said than done, if I could wake her she wouldn't necessarily feed. Also try to get him to feed off one breast for as long as possible to make sure he gets hind milk which has more calories.
It was a stressful time but there was nothing wrong. Hopefully it will be the same for you.