Best formula to supplement with?

My son had his 6m checkup and my dr is concerned as he lost a Oz from his 4 month checkup 😭😭 my supply took a total shit when I pump during the day I'll only get at max 3oz unless it's first morning or he overslept a nap. His dr gave me enfamil neuropro gentlease my son doesn't seem to like it but drinks. So my question is what's the best formula to supplement him with that's like breast milk? But oof that formula is bitter af *yes I decided to try it once my son made a nasty face at it*

Thanks in advance!