You can get a heavier baby spring but tbh you probably don't need it. My DS was also very big (similar percentile!)
As they get bigger they do look more cosy but if your DS is sleeping well then I wouldn't bother. If he is anything like my DS the nest is a godsend especially through teething phases.
However, if you need to then what I would suggest is not moving the nest at all as your DS goes to sleep for a while to let him get used to being still, as that was our difficulty. DS had got used to the movement so a cot was too still! It did only take about a week though and looking back it wasn't as bad as I thought!
As you can tell I loved the nest and so did my DS.