So Mathanxiety, instead of appreciating the sacrifices our forces make for the good for all of us, you see their career choice as selfish and that men like the OP's ex do not deserve much grace when it comes to them trying to maintain some modicum of family life/bonding.
What would you have men who are in the forces who are divorced, re-married and with young children do? What do they deserve? To be castigated? Should they leave the forces? Or is it too much to ask people like the OP, who knew what his career entailed when she married him and had a child with him, to work together in the best interests of the child?
I really can't see what the big deal is. The request is reasonable, the OP should be happy to know that her Don's stepmum cares for her son and wants to spend time with him, that her ex cares enough to make arrangements that maintain his sons routine and family bonding.
Would the OP prefer that her ex not give a damn? Not have much contact with the little boy (neither him nor stepmum) while he is away? Then there would be another thread complaining about him being a terrible father.
Stop using children as opportunities to strike back in an attempt to settle old scores. Think about what's in the best interest of the boy. The OP's son is in a position that many children of divorced parents would love to be in. Three people who care about him, who want to spend time with him, and who currently have an amicable arrangements for his care.
Rock the boat of you must OP but think about the potential consequences. If you think the request would be in some ways problematic or have some detriment for you son, then don't agree to it. If he will be just fine, then let it be.