My son in law just registered a boat with similar set-up, DMV can get you close...the numbering does reflect a date at least year in there...not always month though.
After more then 10yrs dealing w title-less, no registration &/or no HIN boats in person and thru multiple forums, thats the 1st time, ever, any DMV, in any state, used a serial # to ID a year.
The DMV in some states will assign a State issued HIN, and the year its issued becomes the default year of the boat.
There are very few boat makers that have a decodable Ser#, Feathercraft is 1 of the few. Sea Nymph is not 1 of them, that # does not and never will be decoded to provide a year. Perhaps your sons boat was 1 of the few. The vast majority dont.
I've owned 2 Sea Nymphs, 1 pre-72, 1 post 72, and neither had a decodable Ser#. The post 72 did have a HIN, so I know exactly what year it was.