grammatical?
 

a sentence:

His reign was much shorter than the reigns of both his father and his son.

to rewrite it:

His reign was much shorter than the reign of his father.

so, usually I write it :

His reign was much shorter than that of his father.

the following sentence is also acceptable:

Their reigns were much shorter than those of their ancestors.

in these cases, THAT agrees with REIGN, and THOSE with REIGNS.

a question comes:

can the first sentence be written as

His reign was much shorter than those of both his father and his son. ?

it is of course understandable, but the original form seems to be [than those reign of ], without an s, which is absolutely ungrammatical. what do yo think?

Re: grammatical?
 

It's fine. The full version would be "than the reigns". Those here is a demonstrative pronoun and would stand for the plural construction definite article + plural noun.

Re: grammatical?
 

thanks~~