I've got a Moby wrap, but not sure it works for that size and not sure he'd get used to the tying.
My experience with the regular moby on DH who's an XL was that it didn't have much fabric after it was tied. You might look into getting a length of comfy fabric and make your own. Then you can go wild with stretchy or a more rigid material based on what feels best. I briefly thought about this but it's so rare for DH to baby wear I just bought a variety of carriers for me instead!
PS - if the tying is an issue (it was for mine!) ring slings are much more user friendly :-)
My DH is a size 3x and he is able to use the Boba 2g, he had no interest in the Moby. I had him try on a few carriers b4 we bought the Boba, I'm not sure if the 3g which can accomodate a newborn is the same size belts. Try renting a few from paxbaby. Or maybe a custom Mei Tai.
Good Luck. Hope this helps.
Hi! I'm bigger and so is my husband and so far the best we've found is the Ergo (the regular version, not the Organic or any of the other fancy ones) with an Ergo carrier belt-extender for the belt that goes around your waist. It works really, really well! And you can use 2 extenders attached to each other if you need more length. I tried to Moby with our little one a few times but we both got wayyyy too hot for it to be of much use. Anyways, good luck! And way to go, Dad!
Sleepy wrap made by Boba http://store.bobafamily.com/baby-wrap/ is a great "moby-like" wrap and is better suited for plus size people...IMO. I'm plus size gal and LOVED our sleepy wrap from when DD was a tiny baby. Stretchy wraps are only really good until they hit 15lbs though, despite the manufacturer saying they go until 30lbs. They aren't supportive enough.
You could have a sling made for him from a woven wrap and basically have it made in any size you want. I've seen slings made up to XXXXL from a woven wrap.