How did your baby start the sitting process?

My 3 and a half month can roll over both ways from his belly to his back and rolled over ONCE from his back to belly, lol. now when he's in my lap or being held, he feels like he's straining to sit up. he tries to pull himself forward and puts his head forward but can't because his stomach muscles aren't that strong I guess. and if I hold him up to sit, he's sturdy for a second but then wobbles all over. he can sit with his back supported but how can I encourage him to get into a sitting position? did yours roll over and pull herself/himself up to sitting? or do you just place them in that position? I know he's young but it seems like he's trying to do that. any tips are welcome :) thank you!