I'm not overly worried - if I strap him in to his high chair he doesn't slump forwards, and if I hold him on my knee he will sit ok. He has the strength in his trunk, but not the balance.
However, he doesn't want to sit - if I prop him up he just slides down onto his back and rolls around. Hmm. I think the fact that he can't sit is almost certainly because he is not prepared to try
However it would reassure me to know if anyone else has/had babies like this, and when they started sitting (propped or alone)- thank you!