I would think that there's nothing to worry about. A baby's physical development isn't something you can teach. Each baby develops on their own in their own time. Things like sitting up and standing have more to do with muscle strength than learning what to do. Make sure you're giving your son plenty of tummy time during the day so he can strengthen his muscles. There are also booster seats you can get to help him, but if his muscles aren't strong enough a booster (like a Bumbo or Bebe Pod) can cause their muscles to develop improperly. You can mention this to your son's pediatrician during the next developmental assessment.
From your observation, is there anything you think is not right, eg his neck muscle seems weak or he gains only little weight ? By age he already supposed to be able to sit on his own, though every child differs. Still, follow your instincts and ask your paed to do assessments,coz at least if there's something need attention, it can be done early.