My  dad is his in late 50s, he has 0 friends, literally. To describe him,  he is someone who is super proud of himself, egoistic. He considers all  his colleagues lesser than him and considers himself to be  higher than  them. He has good knowledge, atleast in his field.

He  has a colleague who he hates. Probably the colleague hates him too. His  new boss was biased towards his colleague and favors him as the  colleague (Mr. X) butters him a lot. The new boss is probably not smart  and is very scared of his superiors and hence always needs the lower  levels (My dad and his counterparts) with him all the time. My dad hates  the new boss from the beginning as he favored Mr X.

Both  my dad and Mr X were up for promotion recently. Before the result and  interview, my dad had this in his mind that all the nature's forces are  working against him to make sure he is not promoted. At that time his  behavior was very bad, he was angry all the time and a lot. Whenever he  was home, he used to take out his frustration (with words) on my mom.  Actually not on my mom, just in front of us. He would curse others  especially Mr X as he was going to stop his promotion and stuff. i hated  this as my mom would get very sad. He was 100% convinced that he would  not get the promotion and was so sure that he was going to write his  resignation letter as for him "the dumb" Mr X getting promoted instead  of him was something unthinkable.

My  dad was promoted, Mr X wasnt. Now he was not frustrated. Though after  some days, he started thinking that now Mr X now harasses him a lot,  trying to give him extra work and stuff. Basically he thinks whatever Mr  X does is against him and X has a motive against him. So he now gets  angry at home again on Mr X and curses too (less than what he did before  promotion). He is so much into this stuff that he has no interest in  anything else, no interest in renovating the house or making some other  big purchases, or any other home's task or anything to do with my mom.  He is just occupied with his thoughts on colleagues.

From  the past few weeks, he is also bothered about a junior. He thinks his  new boss favors his junior(again buttering) and his boss would have once  said that the junior is good at his work or something. This junior is  not polite and has often cursed my father in front of him. So now my dad  gives extra time at home to point mistakes of the junior and make sure  the boss knows about these.

In  doing all these, my dad would often not eat dinner, skip lunch, skip  lunch parties in office as Mr X would be there, skip drives with boss  and colleagues, would not sleep properly, would take out his anger on  something else, etc.

Just so that  you know, the boss had also recently said to my dad that your work is  perfect and nobody can beat you to it(but he often mentions that my dad  is slow).

What can I do to make his mental health better?