I agree with Karen, I wouldn't push or worry about your bunny not eating soybeans. I've never seen them recommended on any of the rabbit diet lists published by rabbit-knowlegable people.
Mine get unlimited timothy hay (not alfalfa--too high in calcium) and all the dark green veggies they can eat...romaine, green and red leaf lettuces, endive, a limited amount of spinach (high in iron), bok choy, limited broccoli (causes gas), outer leaves of cabbage (not the light inner leaves). Oddly enough they love corn husks. Dandelion leaves are the best if you can find them, also parsley, cilantro and dill are good. Raddichio is good, but expensive, kale, mustard and turnip greens must be limited too as they are also high in calcium. Carrot tops and celery leaves are good too.

Really, your bunny isn't missing anything by not eating the soybeans. You might check www.rabbit.org and look at their list.