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Hello and thank you for your question. This behavior does not sound like a delusion as it would have to be a fixed false belief (e.g. thinking the government is out to get you or some entity is following you and spying on you), so just thinking of his ex is not a delusion, but if he thinks that he is still in a relationship with his ex and no matter what evidence you provide to the contrary does not change his mind than that would be classified as a delusion. Now because he thinks he is going to be together with his ex soon more expresses a subconscious desire or some unresolved feelings for his ex, but not a delusion. Overall his behavior sounds consistent with someone who drinks alcohol to excess as his cognitive processes have changed while he is intoxicated. So he is not exhibiting a psychotic symptom of a delusion while he is drinks heavily, but is more showing a change in his cognitive thought process compared to when he is sober (which happens to most people while they drink heavily) and possible unresolved feelings for his ex.
I hope this answers your questions and gives you some guidance on this issue. Please let me know if you have any questions or concerns as I am happy to assist and support you regarding this issue.
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