Babies do things in their own time but just do lots of assisted sitting with him that way he'll get stronger 🙂
My baby couldn't sit up at 6 months and one week either, I was really worried but at 6 months and 2 weeks he randomly started sitting up. So don't worry all babies are different and do things at their own time, just do lots of assisted sitting with them as someone already said and they'll be fine.
All babies do thing in their own time. Just let him get there on his own 😄