If he's 6 months and 18 lbs I would definitely start solids. It might not cure it but I don't think it would hurt. I will just say not all kids or adults are great sleepers. My DD still wakes up at over two years old. She could have had a nightmare or got twisted in a blanket or have a tummy ache. The reasons for her are endless. It's just who she is and I guess someday she will not cry out when she wakes up. But my guess is that day won't come until she is like 12 lol.
I have done sleep training with her and it doesn't matter. I feel that some people are just crappy sleepers. DH and I both have some issues in that department so I really can't blame her. But I know how hard it is.