An obvious giveaway that I'm surprised wasn't brought up
Teeth
There are few pictures of Nicholas, but in a couple of them you can tell he has normal teeth that don't feature any noticeable gaps.
Fredric has horrible gapped teeth. Granted 3 years of receiving no dental care could hypothetically lead to worn down and decayed teeth, but it wouldn't cause obvious gaps in teeth that have been there all along.
A simple quick check of dental records could have easily proven it wasn't him.