The problem for new mums is, you not only compare your little one with others, but your fmily very 'kindly' do it for you!
So - for the record - boys are always slower at sitting up and walking than girls, <y eldest girl walked at ten months, my wife's cousin (girl) took until eighteen months!
Just have a quick look around your local school's reception class - they are all walking, and talking, and none of them are in nappies!
They all get there when they are ready - it's not a race, so no prizes for being quicker than anyone else.
Ignore the parental 'one-upmanship' - their boasting is nothing to do with them anyway, it's the little one that does all the work.
Your son will sit up, and walk, and talk, and tie his shoelaces and his school tie - all when he is ready.
So don't worry, just enjoy the peace when you know where he is all the time - it doesn't last long!!