So difficult is it to diagnose and treat people we actually see and touch, that it is nearly impossible to say anything useful about someone described on a public forum. Given the information you have provided, I would have to point out that the psychiatrist in question has a therapeutic relationship with your son, a relationship that may well lead him to believe that your son's condition is worsening and may be dangerous for him. The helping professions have become increasingly aware that the biologically based severe mood disorders often begin in childhood or adolescence, rather than in adult life as we were taught some years ago.
If you reflect for a moment on the astonishing constraints being placed on all physicians by the current state of third party payment, and the difficulty faced by any psychiatrist in hospitalizing anyone, the very fact that this psychiatrist wants your son to go into the hospital may well be a good indication of the severity of illness involved and the benefit to be achieved from hospitalization.
Furthermore, although your former husband may have had a relatively benign form of bipolar illness that could be managed without medication, that affords no predictive value in guessing the severity of your son's illness. Although the genetic basis of Bipolar Affective Illness is well accepted, important here is not the presence or absence of a condition, but the degree of illness present in an individual at one particular moment. From the little you have presented here, I would implore you most earnestly to listen to your son's physician, or to get another psychiatrist as a consultant, but not to risk the option of no treatment unless you can get a well-qualified expert to support your choice.