Just on a note I think 120mls is very good... I was lucky if I got a couple on ounces (60ml) out when breastfeeding as well....
I was thinking about this earlier, and because babies clutter feed on a evening and have the majority of their daily milk then, I'd suggest you should give your Mum around 20-30oz, and try off on a small amount at each feed )perhaps 4oz?), and if he's still hungry then she could give him alittle more. I know the majority of babies will take around 6+ oz per feed at about 4 months though.
Like I said previously it's quite hard to attain because all babies are different... but I'm sure it's something you can work out with your baby.
Good Luck, and enjoy your night-off.... x
