6 Week Old Grunts in His sleep...normal?

Burp him, use Mylicon infant gas drops too. Since he may have gas.
Also, after feeding, some babies need to be kept upright, so their 'food' does not just come back out.... but yes, it may be reflux... or just that his tummy is full and it is regurgitating. Also, sometimes as a baby sleeps, they don't exactly 'swallow' as saliva accumulates in their mouth... because when they are an infant, they cannot at-will swallow their saliva like how an adult does, during sleep.

My son was a BIG grunter. He just made noises in his sleep. And sometimes I would see saliva drool by where his mouth was.

But after feedings, it is important to burp the baby...

Him breaking his latch can be due to fast let-down or no let down.... it is the baby trying to adjust their intake. If not enough milk is coming out.... then they do that too, latch and unlatch to try to stimulate the milk flow, as well. Or they cry, because not enough is coming out, or too much.
You could see a Lactation Specialist... and ask them.... so you know specifically, what is going on.

Also, in an infant, growth-spurts occurs every 3 weeks or so. So, your baby may be going through a growth-spurt as well... and needs the intake. So yes, feed him on-demand during the night as well. As you said, he seems hungry.
You also want to make sure, you are producing enough milk... but if you feed on-demand, your milk production should increase per your baby and his feedings.

Also, with infant, they also often "cluster feed" which means they even feed every single hour. This is normal. It is them growing and needing more intake.... both my kids did that too.

It could be a lot of reasons... but, since you are concerned, ask a Lactation Specialist as well as still asking your Doctor about it.
Is your baby growing/gaining weight well? If so, then your milk supply is probably keeping pace with your baby... for example. And if your baby is having adequate pee and poop diapers, this is a good indication as well, that your milk production is keeping pace with him.

Next, there is a difference between 'vomit' and spit-up. Spit-up may 'seem' like vomit.
If possible I would keep him upright after feedings, so his milk & full tummy does not come right out, right after.

LA LECHE LEAGUE.
See their website.
Call them.
Read their book.
Any or all.
S.

Grunting is normal - our son did this for months. It was hard for me to fall asleep as he was sleeping in our room! He also had a lot of gas and I ended up giving him Mylecon drops after every feeding. I think his gas may ease up but I'd suggest giving the drops after every feeding. Also, you should still be burping him - I'd say until he's about 5/6 months old. Even if he does wake up, he'll go back to sleep. What I did was breastfeed while laying on my side, then put baby on my chest and burp (if he didn't burp I'd sit up) then move to the other side and finish feeding. There certainly were times I'd fall asleep (and he would too) so I didn't burp, but I'd try to if I were you.

Also I found out at about 2 months old that he was hungrier than what I was able to give him. I started to supplement and he was eating 4ounces after each breastfeeding!!! So that may be one thing - maybe he's not getting enough.

I suggest lots of tummy time and make sure to get a burp each time you feed him!!!

Wow. Most of this sounds familiar. I can't answer everything, but I have to say the grunting is normal. Ours is eleven weeks and still grunting strong (though sleeping a little more quietly). Mine fusses and breaks latch quite a bit too as my milk lets down (about 30 seconds in). I think she's just sucking like crazy with pretty much nothing coming, and then WHAM! She chokes and gags as milk comes rushing in. She does better with that when she's calm or half asleep, so I try to give her the faster, fuller side at those times. My oldest has reflux, but I think it's hard to tell when they're babies. It's pretty normal to spit up too. I don't really burp mine at night, but I do kind of swing her up on to my shoulder when she's done and rub her back, but usually nothing happens. She eats much more calmly at night, and tends not to break the latch then, so I don't think she gets as much air then.

Just my experience, but I hope that helps. :)

Ahh, the grunting question. My first never did, but my second (now 14 weeks) was a huge grunter for the first 8 weeks. Very annoying when we traveled and he slept in the room with us. But it's normal. And one projectile like vomit is nothing to worry about. If he starts doing that after most of his feedings, then it needs to be investigated. If he farts alot, well then he's a gassy baby. Poor kid, because that is not fun. I second what most of the others have said. Burp him after he feeds off each breast. He definitely needs it. As for breaking the latch, it could easily be your let down. It may be more forceful then he's comfortable with. Try expressing it little bit before you feed him, that should help. And stick with it, he'll work through it and become comfortable again. It'll just may take some time.

Grunting is normal.

My guess re. the feedings is that your milk is coming out too fast for him to handle. Maybe express a bit first before feeding him.

Don't feed him lying down - that makes it even harder for him to ingest in properly, and probably leads to more gas since he is probably swallowing more air. You definitely need to burp him well after every feeding. And FYI, waking him up after he sleeps is actually VERY GOOD! It'll help to disassociate nursing from sleeping, which you'll need to do in order to help him fall asleep on his own without you (unless you don't mind waking up to nurse him back to sleep ever two hours for the next several years !!!!!!!)