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Well, it must be really hard for you and your family to go through all this , but i would suggest that, at this moment all of you should stick together and provide full support and care to your grandson , as from the symptoms you have mentioned , it appears that currently, your grandson is in a state of Mania, which is one of the 2 extremes of mood seen in Bipolar Disorder, the other being depression.
Not holding a job , abrupt swinging of mood , being aggressive , disobedient, hurtful towards his mother and near and dear ones, wandering aimlessly, venturing out without giving any information to you people and probably enjoying all night long with friends , saying words like ---i command you to his mother , which refer to ideas of grandiosity , often seen in manic people, all these features are suggestive of a full blown state of Mania , which i believe needs prompt Attention, in the form of immediate meeting with a psychiatrist and admission in a Psychiatric In Patient Department of a Hospital or Psychiatry Nursing Home.
And , i also think that he definitely has not been taking his quota of drugs that was prescribed to him , and as a result he has entered intto mania. This state of his should not be taXXXXX XXXXXghtly and should not be misjudged as bad behaviour or temper tantrum , as in mania , a person has a feeling of extreme degree of self confidence, feels superior than others, has extreme degree of happiness which is interrupted by episodes of low mood , a person becomes hypersexual, that is there is increase in one's libido( sexual drive), may become disinhibited ( i.e is becoming disrespectful to seniors and elders, snapping or even raising hand over elders, which otherwise one would normally not do) , often manics become spendthrifts and spend lot of money over useless articles and take lousy , unintelligent important career or life decisions.
Above mentioned are some of the features of a Manic Episode in Bipolar Disorder, your grandson need not have all the above features to be termed as manic , as there are variants in this mania , but as it appears your Grandson is surely in Mania, and i would suggest you to Fix an appointment with a Psychiatrist as early as possible and discuss with him/her the possibility of his admission to a Psychiatric In Patient Department , while getting him started on Anti psychotics and mood stabilizers.
I hope you heed my Advise , and take action in this regard as early as possible as we would not want things to get out of control.
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I can understand your concern about the fact that he is an adult and he cannot be forced into seeking professional help , but Since he is a case of Bipolar disorder , and presently in Mania, i would advise you to consult a Lawyer or on just answer itself consult the legal forum, and find out ways in which he can be made to see a psychiatrist , as my expertise is limited to psychiatry and in this case, legal issues are also involved, which can be dealt by someone who knows about the law.
As far as , you not aware of his whereabouts is concerned try calling his friends , and tell them about his present state and the prompt need for medical intervention, before he may do something out of the ordinary and land himself in trouble.
Also, you may seek help from Mental health Institute and discuss with them the possibilities, i am sure they must have had such cases before and would be knowing how to deal with them.
I hope everything works out well and your grandson returns to you.