|If you are okay with possibly having extra cover on him, I would think that you'd be able to make the M/L work. Usually the covers don't have to be particularly snug-fitting, anyway. The covers can be pretty flexible with sizing, and so much can depend on the build of your child. Obviously you'd be out the extra shipping, but otherwise you're not gambling much by trying the larger size first, since it's something you could use anyway.
From another point of view, M/L is the most optional of the cover sizes, as I see it (though I liked having them with my first because of his build, they weren't a necessity and I sold them before DS#2 was out of them because I had plenty of L that worked just fine). Unless you think that your DS is likely to be on the small side, you'll probably be needing to get L at some point anyway. If that's the case, there's an argument either for a) just getting some L now and see if they'll work okay for right now and skipping everything else altogether or b) combining shipping and getting both M and L right now so you'll have a trimmer fit for now and the L for when you need them (skipping the M/L).