I wouldn't worry about the amount of cholesterol in his diet. Cholesterol is actually a very important nutrient for babies because of its role in brain and central nervous system development. According to the AAP, babies should get half of their daily calories from fat until they're 2 yrs old, then their diets can be gradually modified so that they get 1/3 of their calories from fat. So even though it's totally contrary to what we should do as adults, fat and cholesterol are essential for growing babies and young toddlers.
Other great sources of protein, in addition to beans and tofu, include milk, yogurt, cheese (cottage cheese is a big one), and nut butters. Tofu is really easy to mash into other foods if he won't eat it as a finger food and it makes a good sandwich spread too!
It sounds like you're doing a great job ensuring your DS has a healthy diet. If you haven't already read it, the book "Super Baby Food" by Ruth Yaron is a gold mine of nutritional info for babies and toddlers if you're interested!