My boy was eating about the same amount when he was 9-month old. I am not sure if you should give him more porridge ... one thing you may consider is: give him some fruit or biscuit during the day. Your baby would then have a wider variety of food.
My son Myko have the same birthday with your son - 28 Oct 03. He born on 08:03 a.m., who is older? He also have a bowl of congee + a bowl of ABC pasta + 3 bottle of milk ( 7.5 oz, 5 oz & 5 oz) + 1/2 no. of fruit + 2 pcs of biscuit daily. He is around 19 - 20 lbs. Your son is much heavier.
Are your son try to tell you that he is not enough when you give him water after feeding him congee.
小米米,
May be you give him too much bread. 小米 like bread more than congee. May be you can try to give him congee instand of bread. He may have more if he feel hungry.