It will be a lot better for him to be out on grass when he gets gelded. The dirt lot is going to be full of bacteria that might infect the wound. That said, just go slow. If you find he is gaining too much weight a grazing muzzle is a good choice. I've used them before on horses and ponies that tend to founder and they do good with them.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.