My son can sit very firmly, even I push him, he still can sit properly.
I find one thing, if no ppl sit beside him, he can play toys for half an hour or more. Once I sit and play with him, then he immediately lays on my leg and ask me to hug him. So suggested to let him play by himself and train-up him to sit down. (remember to put him sit on the plastic mattress to prevent get hurt)
After attending the playground, my son starts to practice crawling, it is very funny:lol: .....:-( but after he falls out one times, then he stops and needs to practice again next day