He is your mother's mother's sister's daughter's son. That means your mother's grandmother is common to both of your
Your Mother's grandmother, your grandmother, your mother and then you
Your Mother's grandmother, Your grandmother's sister, her daughter, the boy
You and he are the 4th generations from your mother's side. So your marriage will not be under Sapinda relation. Hence there will not be any problem legally. You can go ahead.
But generally as per our Hindu religion you and that boy will be considered as brother and sister as you are both the children of cousin sisters. But the relation is very distinct. It all depends on the family culture and practices. You can discuss with your parents and the parents of the boy and take their opinion also.
If you are in love and not worried about the elder's decision, you both can sit and discuss and decide. If both of you are above 18 years there will not be any legal issue in getting married to that boy.