Milestones

Hey ladies! My little one is 4 months and 27 days old and he's passing all of the milestones except he can't roll from front to back well. He falls over when I try sitting him up on his own, no support after about 3 seconds and he holds his head very well but sometimes it's still floppy depending how he's sitting. Should I be concerned? Are there things or excersizes I can do with him to help? He hates his bumbo chair as he's really tall and has chunky legs so it's just uncomfortable for him. Any advice would be great. Thanks!