Well, it is always possible that he is on the Autistic spectrum.
Honestly, it is easy to look at things from the outside and wonder or even judge. But usually, there are things going on that we don't and maybe shouldn't know. His mom maybe missed the mark with some appropriate behavior but this is not something for you to bring up. Perhaps your parent that is related to his mom could inquire if there is anything people can do to make it easier on him when he argues over toys with the younger kids. More out of empathy and frankly, it's the same skills your own child needs when dealing with peers. I have a son with a developmental delay and as parents, we were instructed to act like kids with him. For example, not letting him win at games, not letting him have a toy and saying that we want OUR turn, adults picking what to play as much as him . . . just like a peer would. So, instill the take turns system just like you would if it were two kids the same age. good luck