It is a combo rug. But yes I do think that it is maybe a bit too small for him because the neck bit is always coming undone . I also do have a fleece combo on under his waterproof combo dona. Do you think that this could be why too???
I might try a new rung on him over the next few days.
Is he high-withered? This last winter I had a rug on the old horse and his withers are high and bony with age, and the rug rubbed it no matter what (the blanket fit the rest of him just fine, even on his shoulder). If it's his withers are so high, maybe you could sew a little pad that fits the shape around the withers to fill in the dip on either side, and take the pressure off the withers themselves? I did that with a saddle pad for this same old horse, and it helped with his saddle, so why couldn't it work for a rug?
I have brought him a new rug and will put it on him over the next few days and see how that goes.
It is defiantly not worms as he is always wormed and is my show horse so he is very well looked after (except this annoying rubbing LOL)