Adoption of stepchild

I was a Malaysian and recently got my indian citizenship. I have a son from my first marriage which was not legalized in Malaysia hence my son was considered child born out of wedlock in Malaysia. We both left Malaysia when he was 2 years old in 2002. We lost touched with his father (last known about in Malaysia) since 2010. My son is now 15 years old. I have since remarried an Indian Citizen in 2010 (legally and by Hindu ritual). I have applied and about to get my indian citizenship this month. My son is still holding Malaysian passport. His schooling has been completely in India hence he also would like to take up indian citizenship and would like to take his stepfather's last name. Since I remarried, my son has been putting his step father's name as father in all school records since 2010.( Previous school documents are showing his biological's father's name). His Malaysian passport is showing his biological father's name in it. Question :- 1) How do I get my husband to be his legal guardian or adopt him. 2) Should I work on getting the Indian Citizenship for my son first and than work on getting my husband to adopt him or vice versa? 3) Since I have lost touch with my ex husband, will that pose as a problem when applying for indian citizenship? 4) If my husband adopt my son while he is still a Malaysian citizen, will that be a problem. Hope someone can provide some advice.