He sounds fine, well within normal ranges. My neighbors 9 month old scooted for a while and then one day started to push up with his arms, then knees, rocking was next, then slowly crawling. Within 2 weeks of trying to get on his hands and knees he was crawling everywhere. Once they can crawl and feel steady on their hands and knees, they figure out how to sit on their own. They do it by crawling and then wanting to stop and turn to see something behind them, and they sorta let one cheek down and their bottom hits the floor and they push with their arms to sit up. It takes time, but not too much once they realize theres something more to moving. I bet by the end of July, he will be crawling and sitting just fine. Then you have to really watch out! He will start pulling himself up on furniture and walking soon...fun times!
EDIT: be sure to give him plenty of floor time too. The more he practises, the better and faster he will be. Some first time mommies just want to hold them a lot, and with so many baby items on the market to sit them in and swing them, some babies never get to touch the floor and learn what they should. Have fun.