Rex mix means he could be part rex and part anything else including new zealand red, and that he has just gotten more of the new zealand genes than the rex genes. So no reason to believe the breeder was being dishonest about his breed or age in this. Mixes can really turn out either way- more like the mom, more like the dad, or in the middle looking like a mix of both. So if your bun is part new zealand, then that could account for his larger size at a young age. Particularly if your bun continues to grow larger over the next 5 months. One good way to know if he's not fully matured and still a young growing bun, is to weigh him once or twice a week. If he's steadily gaining and not looking chubby at the same time, then he's still growing. Another way to tell if rabbits are young is to look at their face and eyes. They'll have a different look to their eyes when they are younger. But you don't have a good head shot so I can't really tell with your bun. If you wanted to post a picture of when he was younger(showing his face and eyes) and a picture of now showing his face and eyes, it might be a better indicator of determining his approximate age.
If he does keep growing, I think you are going to find he quickly outgrows his current litter box. A good large size one that I use for my rabbits, is a mortar mixing tub that I got at home depot. It's 20x27 and was relatively cheap. They also have a larger size than that. Another option for a large litter box is underbed storage bins.
12 weeks is usually when it's considered safe to very gradually introduce veggies one at a time and starting with small amounts, unless for some reason a rabbit might be extra sensitive and there is a need to wait until the rabbit is older before trying to introduce them into the diet. So I would cautiously start introducing them at 3 months old, keeping a close eye on poop changes and signs of digestive upset(stops eating, lethargy, staying hunched up not wanting to move, belly pressing, repeatedly changing position from laying down to sitting up, eye squinting, teeth grinding, sitting with paws resting in water bowl).
http://www.medirabbit.com/EN/Food/Food_main.htm
https://rabbit.org/suggested-vegetables-and-fruits-for-a-rabbit-diet/
If you don't already feed timothy hay(or other grass hay), I would start feeding that as well. Alfalfa hay tastes good to them making them really picky, and it can make it hard to transition off of it and on to a grass hay when they are fully matured. So better to get him used to it now.
