wife seeking custody of child after 4 years divorce

my divorce by mutual consent came through in nov 2011..cos' of lack of career and parental support and logistics constraints had to give the custody of my 7 year old son to my ex husband..wanted to know if i have the right to claim custody of my son after i have sorted things out and have made a career and can also match up to the material comforts that my ex husband can provide him now and in case he gets remarried then is it that i get the custodial right of my son automatically ? please help.