My baby is 5 month 2weeks old, his milk intake is only 3-4oz every 3.5-4 hrs, is it okay? He likes to drink plain water, can he drink as much as 2oz?

No worries. Every child is different. The volume of intake is important but more important is the growth of your child. If he is growing well along his growth curve and meeting his milestones, pooping and peeing well, then his milk intake is fine. Yes he can have 2 oz of water. As long as water is not replacing meals he is fine. Water is wonderful for hydration and flushing toxins and keeping bowels moving.