How old was your child when you started him/her on cereal... did the dr. give ok?

My son is 3 1/2 months old. He was 9lbs 10 oz when born. He only eats total 5 ounces each feeding, usually every 3 hours. It seems like he may be hungrier than we thought. He normally fusses about 2 hours after each feeding, he may be tired but I think he may be hungry as well.... the reason is that yesterday, he ate at 7 and then fussed around 8:30. I decided to see what would happen and fed him again and he ate all 5 ounces again.

My question: I would love to start him on cereal to see if he is more satisfied and stays happier during the day.... he sure does have his fussy times.

The doc at the last visit said ABSOLUTELY NOT. That was a month ago. Do you think it would be ok to start my son now? I know many people do start early, even if it against doc's wishes.

When did you start your child on cereal? Did the doc ok it?

Also, my son is on lactose free formula... AND needs prune juice (doc's orders) to help him have BMs. A woman I work with suggested I start with oatmeal or barley, and not rice since rice is binding. What do you suggest? Is oatmeal or barley ok for lactose intolerance?

THANKS so much for all your help!