Why are you bothering with formula? (I forgot if it was mentioned elsewhere in the thread...) The very best nipple are on momma.Fockin' nipples!
I've purchased several that claim they have a slow flow rate appropriate for a newborn, but my son winds up choking on excess formula! In fact, the slowest flow rate nipple I found is actually or 1 month olds, but, yeah, there's a but... I can't find the damn bottles for that nipple, so I've had to rig them to work with our bottles.
Fockin' nipples.
BTW - if he starts choking on milk or formula, lift his arm over his head. If he's on his left side, lift his right arm, and vise versa. I dunno why it works but it does.