Regarding my son. Please help me.

Dear doctor,I am in lots of worry. My son was born on 20 th Jan 2016 and 6 years old. When he was 5 months old i started solids and I breastfed and I gave only little amount of water.during his ninth month he had severe diahrrea and admitted in hospital due to infection for 3 days.then i stopped breast feed but daily he took lactogen,fruit juice but still hesitated to drink plenty of water. Now he is 6 years old but only 18 kgs and very lean compared to his friends. I am really feeling guilty that whether lack of water is the reason behind his growth delay. He is highly active and brilliant but physically lean. Whether if he had not got enough water any signs that will appear. I am really frustrated please help me.