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From a relationship coaching lens, some people move on rather quickly because it is how they choose to try and forget. I wouldn't assume that he has fully moved on, especially mentally.
In some cases, people move on very quickly after a break-up because they had already moved on mentally before the break-up occurred. I just had a client that wanted to divorce her husband for 18 months before they actually separated. She was dating someone else within 30 days of the divorce but 19 months after she mentally moved on.