keeping on-call job at Kaiser.

My son is an on-call EDS employee. He has worked for Kaiser now for over a year, but has not been used much. As a result of not being used he has stopped submitting a work schedule for over 6 months. So he has kind of given up on the job but has not advised Kaiser of this.
So now circumstances have changed and he would like to continue working at Kaiser but is wary as to how to proceed in letting Kaiser know that he wants to keep working and wants to start submitting his availability schedule again. Since he is on-call maybe that is why he has not received a termination letter for abandoning his job, or possibly they are waiting for him to show up or to speak to his mgr so that they can terminate him? So we are not sure how to proceed. Possibly he can talk to his Union Shop Steward first? Is there anyone on this forum who has some advise on how we should proceed, or who has faced similar situation?