introducing solids

I’m looking for guidance on how much and how often to feed solids. The pediatrician had me start my 4 month old on rice cereal. I know that’s a bit young but his weight has been low on just breast milk and formula. Right now he’s just getting a tablespoon of cereal with breast milk twice a day. The doctor also mentioned puréed/babyfood fruits and vegetables. Would that be in place of the cereal or in addition? And I assume the amount should go up as he gets older/bigger but I don’t want to overfeed him. I also know it’s supposed to be primarily milk/formula until a year but I don’t want to be too stingy with the solids.

TIA