research "virgin gut". my baby won't be getting anything but breastmilk until she's one. lol as @cubanese said, my baby was also holding her head up at 2 weeks. nothing to do with her digestive system.

If baby shows an interest in the spoon, or seems to need more. You'll know best for your baby! My daughter started at 4 months and her doctor approved based on her having good head control

I'd say talk his pediatrician. I started to give my son cereal at 5 months but he could sit and hold his head on his own by then. and I was never told cereal was bad either, just to be careful. but talk to your ped. I spoke to mine before giving him any