Hello Rose, I'd like to help you with your question.
It sounds like your partner may have been more affected by his father's loss than he may realize. Although he was too young to feel the loss of his father directly, the reaction his mother had may have created anxiety for your partner. He did
not experience his father's death, so he did not get to react to it himself and see that grief can be experienced and worked through. All he saw was the affect on his mother. And the impact it had on his mother may have given him the impression that a loss is an intolerable thing that takes over your life (the hoarding and anxiety she feels) and there is no way to work through your grief. So he has transferred these feelings onto your relationship, particularly focusing on losing you. If you are out of range and he cannot be sure you are ok, then he may imagine the worst which is losing you. It triggers his anxiety about how his mother handled her loss and he has trouble coping.
It is very often difficult to pinpoint this type of anxiety. Your partner cannot say something happened to him directly but something did happen because he was exposed to grief and unresolved anxiety through his mother. It can be likened to the children of Holocaust survivors who "transferred" their grief, anxiety and sorrow to their kids, some of whom developed behaviors because of what they heard from their parents. The Holocaust did not happen to the kids but it still affected them.
Your partner can work through how he feels by first recognizing where his fear comes from. Talking about how he feels regarding his father's death and his mother's reaction would help him identify how he was affected. Then he can either talk to a therapist to help him change how he thinks and feels about what happened to him, and/or he can try working on it through self help. The ideal option is to talk to a therapist so he can get support and also so he can take this issue out of your relationship and work on it individually. Not that you can't help him with it by offering support, but he needs to address this on his own.
To find a therapist, he can ask his doctor for a referral. Or he can search on line at:http://therapists.psychologytoday.com/rms/
He can also learn more about grief and what reactions are normal and which ones are not. Here are some resources to help him:http://www.helpguide.org/mental/grief_loss.htmhttp://psychcentral.com/lib/2006/the-5-stages-of-loss-and-grief/
I hope this has helped you and your partner,