J.,
Go with your gut. I exclusively breastfed all 4 of my children. I never rushed into solids...they don't need them in their first year at all, so why rush it? Eating solids is just practice at this point, and since your son is a preemie, I dare say that 6 months is way too early to be starting solids with him. What is his age when you consider his due date, 4 months? If so, I would wait at least another two months before attempting to introduce solids, and then I would start with bananas and avocados...the best first foods. I never listen to doctors advice when it comes to feeding...so many of them are still of the opinion that you should start solids early. The fact that your son is having increased gas (burping) would be a red flag that his digestive system is not ready for solids.
Oh, and my first child, also a son, also nursed for 40 minutes at a time. He also nursed every two hours during his first 5 months, then went to every 3 hours, so I know a thing or two about what seems like constant, forever nursing. You are doing great with him! He is nursing less because of the solids. I would think that especially because he is a preemie, you should just keep focusing on nursing with him.