My Baby boy will be 1 in 2 weeks time but still can't sit up in his own.He basically lies on his mat & kicks his legs, or he goes in his baby walker & gradually slumps forward, although he does move backwards in it. I also put him in his highchair & he gradually slides down & i've got to keep sitting him up straight. I've got another 2 children who were late walkers but they both sat up without any problems.
Any ideas would be greatfully recieved