If he is doing fine on Oxbow and you can afford it, then stick with it. If it aint broke, don't fix it.
The rabbits I have on Oxbow do quite well and I have been complimented on Korr's coat.
Do some research on the pricing. I can get a 10 pound bag for $15 and I have a friend who got a 50 pound bag for $75, so it works out to be the same cost per pound. There would be little reason for me to buy a 50 pound bag if there is no difference in price and I would have to go somewhere else to get it.
Also make sure that the formula is the same. I think that Martins 50 pound bag is a different formula than the smaller bag and it does (or did last I checked) had animal fat which really isn't good for rabbits. Just make sure you know what you are getting if you switch and go for the big bag. I also know the Martin has changed their formula for the alfalfa and timothy pellets, so make sure they are suitable for your rabbit.
I would put the pellets into smaller ziplock bags. Even if you don't freeze it, having it in the smaller bags will keep it fresh longer. If you just keep opening the big bag, it will go stale faster. Get large bags so you don't need as many as you will go through it fairly fast with a large rabbit.