can a custody agreement be changed after being in military for two years?

My friend has joint custody that was set before entering military. Since entering military, his ex-wife doesn't think he has any rights to visitation, since he's not there to get son every other weekend (which is what was in divorce agreement, which happened before joining military) He wants to be able to see son when he comes home for a visit before being relocated again, and she says no. He does pay all child support, plus more. Can she keep the son from him?