Cereal in baby's bottle?

My son is breastfed, 4 months old, and is eating like crazy! He's a big baby, 18lbs 26in. He's ready for solids but I want a confirmation from the Dr on Thursday first. Well he's eating so much milk because he's just not satisfied. He will eat 8oz easy. If we try abd give him less, he screams. So I bought oatmeal baby cereal to add into his milk. How much should I put in? My mom did this with him when she was babysitting and running out of milk, he liked it but got a little constipated. How can I avoid that?