My ds is the same age and just a bit bigger (closer to 25lb). Usually I use the maya wrap when we're at the store or something, I can get about an hour out of it before I'm uncomfortable. Typically it's not even my back that starts to hurt, it's that he's putting pressure on a nerve in my hip and my leg starts getting numb.
I also use the rebozo, with the same positions. Normally a hip carry and occasionally sliding him to the back, but we're not too comfortable with that yet. I'm finding it easier lately because he is so mobile all of a sudden, and very interested in watching everything. This means at home I can put him down more, and at stores he rides in the buggy when I get tired. Dh and I take turns a lot too.
I'm thinking of getting one of those long stretchy wraps soon. I think it'll be easier to have his weight more evenly distributed, and I think I'll feel more secure with back carries having all that fabric to play with.
Maybe some sort of backpack would be more comfortable for you? I only ever tried a cheapo crappy one and it dug into sensitive spots on my shoulders, but I could see how a good one would be helpful.
I haven't seen it yet either, but people have also told me it gets easier when they are older and help more by clinging to you. Can't hardly wait for those days!
So I don't know if I've been helpful to you at all, but that's how I handle a big baby.