Is this the normal course of development?

My baby boy is 6 months (and 3 days) old...he can roll over both ways and keep his head up well while on his belly...he tries to sit up when I keep him on the his car seat (reclined) and can sit like that for a while and then will lean back...I still give him head support when I carry him (even though the doctor told me not to ) I'm still worried his head will fling back...hes healthy - weight is ok - doctor is happy with him...I'm not worried or anything but its just that I have a cousin who has a baby girl (whos 12day younger to him) and they dont give head support at all and she seems fine - said she even sits in her bath and I've seen my cousin keeping her seated in her lap without any support...Shes a cute little baby too and I dont want to compare...I'm just curious