Constipation for last 1. 5 years.

My baby is 2.5 years old. He passes stool daily sometime once in two days. But it is not easy for him to pass stool. It takes 20-30 minutes running sitting dancing for stool. So i went to doctor he prescribed EvaQ but no result found. Only consistancy has been little bit soft. But same habbit . He usually do once in two days in every week. Also he has behavioural problem, speech delay, aggression, secre tantrum. Can constipation cause this. Because it's his broblem for more than 1 years, we thout it's his routine but now understand it is something serious. So my doctor only kept saying increase quantity like 10 ml twice a day. I have been giving Eva Q for last 15  days but no improvement. What can I do next . What could be the main issue because of this he hasn't gain any weight in past 1 year. What could be the possible cause or go for blood test if any liver intestine problem. I m serious concerned. An in my city doctor doesn't want to go in depth he only say keep giving medicine.