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You should contact the American Red Cross. They have the ability to contact soldiers while they are in training and give them information. That information can also be given to his command.
Now, all that said, unless his father passes away, it may be unlikely for a commander to release him. If your daughter is actually unable to take care of herself (medically unable) and only the grandson can do that, then command might release him on a hardship tour. Command can also certainly choose to let him go for short term leave, but what you have to understand about military service is that commander decide when you get to leave, not circumstances.
If his command won't release him, he won't be able to legally leave.
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