Not sure on the fill of my horse's rug?

So when I bought my horse last year he came with (along with all his tack) his winter rug. Later on in the year I ended up buying him a new summer one but now as it's starting to get colder as it's nearing winter, especially as he had two viruses/bugs in the past couple of months, I need to get him into some sort of "autumn" rug, something a bit warmer that isn't quite as heavy as his winter rug, which I don't want to get him into yet because it's nowhere near as cold as it will get then.

So I want to get him into a rug with less fill than his winter, but obviously something heavier than just the unlined canvas summer rug he's wearing at the moment (yes I'm aware it's not summer anymore but up until the last week or so only the nights were starting to get a bit colder and very recently have the days started to get colder too), but his winter rug has no labels left on it anymore and the place I bought him off (long story, but he still grazes there, he's an ex-therapeutic riding horse) hasn't got a clue what the fill is either.

So is it possible to find out what the fill is? I have no idea what rugs are available in other parts of the world but if it helps at all and anyone knows of these sorts of rugs it's a Shires Stormcheeta, kinda a green/brown colour (I had a quick internet search to see if I could find it since it's not a brand new rug anymore, but no such luck yet...). Failing anyone knowing, is there a known way to do it by weight or something (or some sort of other method of identifying it by rug features)?

Like I said, I don't want to put him straight into his winter, nor do I want to buy him a new one of such a similar fill that it defeats the purpose of having it as an inbetween rug.

Thanks in advance if anyone knows, failing all this I'll probably just go buy one that feels lighter by heavy enough or buy something I can layer with the canvas (which won't be perfect but will make it heavier and keep him a bit warmer - though go figure he's not some sort of lean breed, he's a stocky Stationbred).