You need to find out more info. You stated he "upgraded" some years back. If he only owns weeks, that likely means it is enrolled with Abound. (Or he bought additional points.) If not, he can enroll for free as he has owned 20 years. Enrollment makes those weeks much more valuable. Also,
you can lock off to 4 weeks and exchange for free.
He owns platinum but you did not say the view, which is either oceanside (OS) or oceanfront (OF). The weeks would yield the following abound points:
OF 5375
OS 4325
You also can rent them out for about $6,000.
For 2 OS weeks it would cost over $50,000 to buy those points resale, and over $100K if bought from developer. So, yes, the weeks have a lot of value. As his child, enrolled status can be passed down to you.