He can not join the military full time if he has primary physical custody. He can (and should) maintain joint legal custody as well as reasonable visitation depending on where he is at any given time. In SOME states, there are laws allowing other family members to exercise the military member's visitation time while that parent is deployed, but that only applies to time that would be court ordered as that parent's visitation time - so every other weekend, or a week in the summer, or whatever the court order says - and not all states allow this. However that is NOT the same as custody (you will never be allowed to make decisions for the child) and it will not be primary placement, and it would have to make sense given the distance between you, mom, and dad's primary station. If dad lives near mom and has every other weekend visitation, but you live 300 miles away, it would not make sense for you to make the children travel that far, that often. All parties need to keep the best interests of the children in mind at all times.