too many variables.
If his mother was impregnated on Jan 1st then he would be born roughly 9 months and a week so if your mother was impregnated on Jan 1st then the odds greatly increase but it all depends on the woman and since his mother and your mother do not share any genetic traits(which does not necesssarily help in determining how long a baby will be carried) I think it's just luck of the draw but the odds do increase with both women getting pregnant in the same time frame( 2 weeks)