Yellow, I can't really help, I don't think, sorry. It's a bit outside what I know about. It's good you have the medical input.
Maybe the Enfamil is thicker and stays down better, as you said. Maybe he has a very sensitive valve at the top of his tummy and this means he throws up a lot (this is not anything to worry about in itself - plenty of babies have this and they grow out of it.).
The thing that sticks in my mind is that he lost 7 ounces in weight - is this now reversed? For a baby to pass blood due to constipation is something to be taken quite seriously, I would have thought - the milk cannot be suitable for him if it has this effect, I agree.
It would still be fine to try him on the breast - he certainly won't get constipated with breastmilk, and it doesn't matter if he chucks it up. Like I said on the other thread, all you do is put him to the breast as part of a cuddle - honestly, it would really be worthwhile thinking about going back to breastfeeding because i) that's what you really wanted to do ii) formula is making him have problems
You would need to keep him on formula until the point at which you re-built your milk supply....but even if he only has some breastmilk, it might help.
Good luck with this.