Baby not gaining enough weight πŸ˜”

Jarinil β€’ So in love with my baby Iris β˜ΊοΈπŸ’•- born 3/23/15
As much as I feed my girl on demand she still only gained 2 oz this past week! 😩 it's supposed to be .5-1 oz a day...so I have to supplement with extra pumped breast milk. I'm happy with that but my husband keeps saying we should go to formula because I'm not giving her enough...I feel so bad. 😞 Has breast milk supplementation worked for anyone else to chunk up their baby??