Hi there
I know your LO is another month off from solids - if you haven't put it all in the bottle (ie milk he has not touched although you know it's a bag he is refusing, maybe a small amount to taste test first??) you could cook with it and then freeze the cooked food in readiness for starting solids. I'm thinking sugar free milk pudding squares/fingers, sugar free custard, white/cheese sauces you could stir through pasta. You can use it in any cooking/baking - sugar free mini pancakes are a finger food favourite and can be frozen interwoven with baking parchment so that they don't stick together and you can lift out one or two at a time to defrost, or a batch of sugar free mini muffins perhaps. Although don't use up all your freezer space!
Maybe the other thing you could do is to offer the fresher milk for now and leave the slightly older milk until you start solids and use in cooking?
Looks like the scalding before freezing is a good practical solution to avoid this problem over the coming months.