Hi,
I'm not sure about your son's back situation, though if my children ever slouched I used to straighten them up and jokingly say 'Come on let's sit with a straight back please.' It didn't happen very often so not sure that I've been much help there. With regards to the crawling, in my experience children will do things when they are ready. My son used to love standing, even as early as three months he would stand on my lap, with me supporting him under his arms, and bounce his legs up and down. When it came to moving he didn't budge at all until he was 12 months. He used to stay exactly where we left him and showed no desire whatsoever to move. Once he was a year old he started to crawl and then walked at 13 months. My daughter on the other hand crawled at 10 months and walked at 12 months. Children are all very different and reach all of these milestones at different ages. As my husband said at the time, if you think walking is one of the hardest things you have to learn in your life. It takes alot of brain and muscle co-ordination, that as adults we take for granted. I really wouldn't worry about the crawling, you son will do it when he is good and ready. My friends son didn't walk until he was 2 years old. He shuffled on his bottom everywhere and was efficient at getting what he wanted (so why walk?) OK this is an extreme case, but he is now walking and running just like other children his age. Your son will get there.
I hope that helps.
Jools