Frequent urination in day time and sleep

My son is 2.5 years old. Having constipated in nature even many medication tried as per advice. But no improvement . But there is big problem over this. Urinating problem. He passes urine frequently in day time. And he doesn't tell us that he is going to pass urine. He passes every where on bed floor kitchen. He passes in sufficient quantity every time. Atleast 20 times a day. His weight is also low only 11.5 kg. He hasn't increased his weight for last 1 year. His height is above among his age child. But very weak single ply. Also cause speech delay tantrum . But constipation and frequent urination is top cause. We have given him lactulose syrup over 1 month still he didn't pass stool daily. Help me. What test should I do to rule out any internal body problem like kidney liver etc disease. And for constipation too.