Weight gain and supplementing

So Ari is a little over 11 weeks now and weighing 9 pounds 14 oz. the dr has been saying he's underweight since about a month old because he's not gaining fast enough. He was born 7.8. I ebf, he nurses every 2-3 hours and since I've been back at work I pump and he'll drink 3 oz. he says he's still not getting enough calories and is afraid that he will stunt his growth by trying too hard not to lose weight?? Idk, never heard of that. So he wants me to start supplementing with formula. I'm just so afraid because my 3 year old didn't do well with formula and wound up on miralax daily and still got impacted twice because he just couldn't poop. I don't wanna have those same issues with him. Any ideas? If I do give formula, could I mix it with breast milk instead of water so that it's easier for him to digest? I tried enfamil gentlease and newborn and he didn't poop for 3 days after having it and just seems very fussy and gassy