My son will be 5 months old on Jan 17th and is currently 19 weeks old. I'm a little worried about some of the milestones he's meant to be hitting... For example he can't sit at all unassisted, he's very off balance but has the physical strength to hold himself up. He also doesn't crawl. He's a very large baby, in the 98th percentile and his head is gigantic so I'm wondering if that may be affecting his balance. What do you think, should I be worried? Thanks