Advice on sitting up

My son is 6 months old and no where near sitting up yet, he sits for a second then faceplants, or tips so far forward his belly is contorted and he vomits.

What do we do to help him? Clearly tummy time isn’t doing enough to strength his back and neither is sitting in his bumbo chair. I’m worried he’ll fall behind developmentally (he didn’t start doing tummy time til 4 months due to excessive screaming so... we had to play catch up with that)

Do we just... let him vomit and face plant and teach himself? I feel like if I keep lifting him and holding him, he’ll never learn or his muscles won’t strengthen.