Hi, I'd like to help you with your question.
It sounds like your son has associated his Gran's illness and death with eating and controlling his bowels. At the age of 7, he was vulnerable emotionally and in order to handle the situation, this is what he did to cope. It is a not unusual for phobias and fears to start this way. And there is a positive solution.
He may feel like something bad will happen if he does not control himself with eating and moving his bowels. But in truth, he is trying to control his feelings. What he needs to do is address the feelings his Gran's illness and death caused for him. He may not have been able to express his fears when he was 7 years old, but he could explore it now in therapy.
Do you think he would be willing to talk to a therapist? It is the best way to cure anxiety disorders. Medication is great to reduce the feelings of anxiety, but it does not get to the root of the problem and solve it. Let him know that cognitive behavior therapy and integrated therapy both are the most promising of treatments and often greatly reduce or cure anxiety. Have him talk with his doctor about a referral, or if he attends church his pastor could help. Also, if there is a local community mental health center he could make an appointment there. They will do an evaluation and determine what his diagnosis is and they will probably offer outpatient therapy, one hour a week.
For now, at home, he can start helping himself by not fighting the feelings that come. It is hard, but he can do it with practice. When he starts to feel the anxiety come on, let go. He can tell himself there is nothing to be afraid of and keep repeating it until he feels better. Practice not going to the bathroom as often and eating slightly larger meals. The idea is to face the anxiety and see that nothing happens as a result. He can also educate himself on how the digestive system works and this will help him realize that he has control. What he needs to do is realize his feelings are about him losing control and that he is using his bathroom habits as a way to deal with this control.
While he is working on letting go of his anxiety, he can also learn more about anxiety so he understands why he feels the way he does. He can also learn better ways of coping and techniques on how to help himself. Here is a list of books I highly recommend:
The Anxiety and Phobia Workbook by Edmund Bourne
When Panic Attacks: The New, Drug-Free Anxiety Therapy That Can Change Your Life by David D. Burns
The Cognitive Behavioral Workbook for Anxiety: A Step-by-Step Program by William J. Knaus
Coping with Anxiety: 10 Simple Ways to Relieve Anxiety, Fear & Worry by Edmund J. Bourne and Lorna Garano
Also, try Lucinda Bassett's program at the Midwest Center for Stress and Anxiety. You may be able to find it through your library system (it is expensive otherwise) and also she has several books that are very helpful. Claire Weekes also has many great books on anxiety that you may find helpful.
Have your son keep in mind that anxiety is very common and is highly curable. There are numerous techniques that are out there that he can use to help himself recover and be able to deal with his symptoms. He can feel better, and soon.
I hope this has helped you,