Solids?

My LO is turning 5 months tomorrow and showing all the signs of starting solids. He’s sitting up almost completely on his own, has been holding his head up since about 3 months, is trying to grab our food, and is making chewing sounds when we eat with his mouth. Dr. Said it was fine at four months but I was leary. When did everyone else start??? He’s currently drinking 7oz of formula every 4 hours. Was thinking of starting him on cereal for 4 days.