He's only 8 mos old. My daughter just started crawling at 10 mos. He'll probably be an early talker. If your pediatrician says he's fine, then he's most probably fine. I don't think its that big of a deal if your son doesn't walk by the "average" age. Your dr said his legs are fine and he kicks them. He's doing well in all other areas.
Don't let the in laws get to you. Your his mother and you know best. Your pediatrician would need to refer you to a specialist anyway, so just tell them that your dr told you to wait a few more months before going to a specialist.
Here are developmental milestones:
Notice it says,"MANY babies like to stand and pull themsleves up by 9 mos". The operative word being many. Not ALL.
My nephew never supported himself on his legs until he began to walk at 10 months. He is now 21 mos and very active.
Every baby is different. You said yours is laid back so before you begin speculating, respect his pace and wait it out a bit. He's just taking his time. Your pediatrician will tell you when it becomes a problem. Good luck. :-) I'm sure he's fine.