serious in-law issues

To the OP Riya. Here's what you can do while you are in US:

Take this as an opportunity to be away from your IL's and try to bond with husband. Plan weekend outings regularly with hubby and son and make the most of that time so all three of you enjoy it. Let your husband feel how you and your son are his real family now and what it means to enjoy with wife and kid.
Make it a regular habit to go for evening walks with husband and your son and enjoy the surroundings, carry a camera with you and click pictures of the threesome of you whenever you see a good spot outside. Make this evening walk time such a cheerful time that once you are back in India your husband starts missing you in his evening walks with his sister.
Make dinner time a family thing with all food prepared and hot and you sit down with hubby to eat with your son too. Eat from his plate and feed your son from his plate to. Make it a loving affair and tease him saying eating from same plate makes intimacy stronger(show him eating from one plate is a husband-wife affair). So that when he is back in India he may find t unpalatable to eat with sister in one plate.
Make friends with other couples if there are any in your area, especially with those who seem to have a very good bonding between them. Invite them over and go on camps, outings etc. with these couples. Let your husband see what it means to bond with a wife instead of sister and mom.
If your son is more than a year old, find out child development classes in your area(continuing education schools too have it sometimes for free) and take him there and let him enjoy and have a good time. Then talk about it to your hubby in the evenings how you and son enjoyed and sonny interacted with other race kids. Let your husband feel that your son might get really good opportunities here in US. He might start on thinking on lines of staying here a little longer for his son. Take extra care of how you look and dress up and exercise regularly and stay fit. Show your husband how happy you are being here and what he had missed in India when he ignored you for mom/sister.

While doing all this never try to talk against your in-laws. If the topic of returning to India comes up ever tell him yes you understand that being the eldest child he is responsible. Don't ever let him feel that you want him to continue staying in US instead and never talk ill of in-laws since that will spoil this fortunate break you have got from them to build your relationship. Don't think of divorce at all. You can workout your marriage and be successful. Just don't give up. Read all the advise given here and implement it. I hope you this was of some help. All the best to you and take care.