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DearCustomer- Well I have no legal way to tell you how to force him to go for joint custody. What he should at least do is to get court ordered visitation even if she has sole custody. Once he has court ordered visitation she will not be able to move since it would violate his visitation order which would be for every other weekend plus an evening during the week which is the usual court order.
The two of them can agree on any further times they wish but if he has court ordered visitation she can't move across the country with the kids since it would interfere with the visitation order. With no court order he is at her mercy. At least you should be able to convince him to be certain she can't keep the kids from him at own discretion.
There's only so much you can do to help someone who doesn't want to help themselves. Your son is an adult and it is ultimately up to him to make these decisions.