I add rice cereal to my 13 month old's bottle at night. I also thought it made her sleep longer. She still wakes up twice to drink. At 3 months she was eating every 3 hours so... my opinions is he just a baby and needs to drink often. He's not an adult. He's growing and needs milk. He does not have the requirements to digest solids food yet and I think there was a correlation in food allergies if you force them to eat early. I waited until like 8 months as I am super paranoid.
But good for him! My baby at 1 year was only 18 pounds and i think in the last 3 months only gained 2 pounds. Kids are all different. Mine has a budda belly and checked out by the doc as just fine
She's was a preemie though.
I also add baby rice not oatmeal as rice is the least allergenic and oats might be kinda hard to digest. I know I can't digest them.