He's been rolling for about 2 months but when it comes to sitting he's not interested. You prop him up & he slopes back down. He can sit unaided for about 3 seconds did before falling to either side. He's off the chart for weight & he's on the 99th for height, he's long & a bit chunky but not hugely so. My daughter was sitting unaided by 6 months so I'm comparing (rightly or wrongly). Is this 'normal'?