Personally, I don't suspect that's the reason for the lower score (Unless of course your CR say it is) though I could be wrong. My mortgage has been bought out twice in 4 years. I haven't noticed though if it says closed or not. Not not a closed account, removed, will change scores. Again, I could be wrong. How many points we talking here?
Maybe dispute it as a duplicate account.
Dispute it as having incorrect information IF it states it was just opened the month it reported.
Which website do you use to check his credit scores?