Advice on my 3 month old??

My son just turned 3 months and he is trying to sit up! As in, he will lay flat and he is lifting his head to his tummy like a sit up. His definitely NOT safe to leave on the sofa or anywhere now. (We have a large sofa that was safe before now)

My daughter did this at 5 months so it's weird how he is doing this now. Literally every time I lay him down he does this and it's great but scary. He's getting frustrated and wants to sit up and see everything.

What age to babies sit up on their own? My daughter was at 5.5 months.