If the parents of the child are under a court order regarding visitation and custody they are subject to it's contempt powers. If that is the case, you could petition to court alleging a violation of that custody/visitation order. If there is no order, you have a big problem that 3 months can't solve. Alternative is to contact the boy and send him the ticket.
I think that there are several issues that need to be considered here. The first relates to how your fiance's son is doing in school. If your fiance has shared legal custody, which is typically the case, then he has the right to contact his son's school to find out how he is actually doing to determine if this is just being used as an excuse. If he is actually struggling in school, I would suggest that you propose a plan for the wedding trip that would minimize the amount of school that he would miss and propose specific strategies for how you plan to deal with finding time for him to get the work that he is going to miss done while he is with you or when he gets back home. It is important that you understand that his ex is likely having a difficult time dealing with his remarriage and try to be considerate of her feelings, if you can alleviate her concerns by coming up with a plan, it is in everyone's interests that you do so. I would suggest that your fiance consider reducing his plan to writing and requesting permission from his ex one more time. Give her a short time frame in which to respond so that you have adequate time to seek the assistance of the Court if needed.
Where you go from there depends on what sort of custody and visitation rights your fiance has. If his ex is violating the present order by denying him visitation with his son for the wedding, then you can go to Court and seek to have her held in contempt and to request an order of the court allowing the trip. If the current order does not specifically allow him visitation that would include the wedding weekend, then you will need to take a different approach. You will likely need to file a Complaint for Modification of the current order. Typically these types of proceedings take time, but you can file them on an emergency basis and seek an expedited hearing in cases where there is an exigent circumstance. If you are out of state, your attorney can request that your presence be waived and that you ex be allowed to testify via telephone conference or affidavit, but the judge may require him to appear.
I would strong urge you to disregard the comment above about sending a plane ticket to the son directly. Depending upon the Court order that is currently in place, this may constitute contempt of court. More importantly though, it will place his son in the middle of a dispute that he should not be involved in and that he has no reason to know about. The Family and Probate Courts frown heavily upon parents who involve their children in their custody and visitation disputes. While it may be tempting to do so in the heat of an argument, it is always better in the long run for you take the high road and leave the children out of it.
I wish you the best of luck with your upcoming wedding and sincerely hope that you are able to work with your fiance's ex to find a way for his son to be there with you on that day.
Please understand that this answer is provided for informational purposes only, and it is not intended as legal advice. If you want legal advice, you should contact a Massachusetts family law attorney and provide them with a copy of your current custody order.
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