My girls both sat up at 5.5 months but DD1 didn't crawl until 10 months. One of my friend's little girl crawled before she sat up. I don't think she sat up until about 8 or 9 months. Just see how he goes. It sounds like he's trying and his muscles just need to get a little bit stronger before they can support him fully. Slouching is pretty normal to start with at first too. I wouldn't push it too hard. He'll get there. If you are worried, talk to someone who is qualified to comment. No offence at all to the gymbaroo person but I wouldn't be taking comments from them as gospel x