My son will be 18 months old in a weeks time. He has not started walking independently. He can pull up to stand, cruise along furniture, walks by pushing chair. Once he loses the support, he sits down in a second. He was slow in achieving sitting and crawling milestones. His peadiatric therapist had suggested few exercises at home. But he is not cooperative to do that and we are bit scared to force him. What are other solution? Is there anything to worry?