Constipation problem. For his age!

My son is having constipation problem and we have used constipation relief syrups for nearly 2.6 yrs. Doctor decreased dose gradually and said to stop recently. It has been 15 days I stopped giving syrup to him. My doubt is should i give 8 glasses of water daily to my toddler . Does it cause over drinking of water to him. If I give him lot of water he is peeing for every 30 min. My doctor said give lot of water, fruits, veggies, fibre food. Does eating fruits daily for 2 servings in children cause diabetes. Is eating fruits daily healthy.How to overcome constipation with out giving lot of water as he is toddler does frequent peeing cause losing of minerals in body. Should I stop giving him 8 glasses of water does it over burden kidneys for his age