Where is the verb in reduplication of adjectives "Noun + Adj + Adj + 的"? I understand the 的 from the structure:

Adj + 的
Example: 高的。

as attaching the Adj to an omitted Noun. This omitted Noun can either be a general noun such as &quot;one/s&quot; (高的 can mean the tall one) or an implied Noun from the context (if the topic is a boy, 高的 could mean a tall boy).

I have come across the following structure for reduplication of adjectives:
Noun + Adj + Adj + 的.
For example:

他的儿子高高的。
His son is tall.



Question: Because Adj + Adj + 的 is attached to an omitted noun, it becomes a nominal phrase and grammatically Noun + Adj + Adj + 的 is equal to Noun + Nominal Phrase. Where is the verb and predicate in this sentence? Is the verb copula 是 omitted but implied in connecting the nouns：Noun +（是）+ Adj + Adj + 的？From the example: 他的儿子（是）高高的？

Edit: My original post addressed the question focusing on the function of 的. However, the question of why 的 was necessary in reduplication of adjectives, instead of (Adv of degree) + Adj + Adj, was already asked in Adverbs of degree with reduplicated adjective phrases?
EDIT:
blackgreen's answer reminded me that the noun in “Noun + Adj + Adj + 的” is not the subject of a [SVO] sentence but the topic of a [topic + comment] sentence.
There are two ways to interpret which is the topic in 他脸红红的

[他(的)脸 (topic)] [红红的 (comment)] = [his face (topic)] [is very red (comment)]

[他 (topic)] [脸红红的 (comment)] = [he (topic)] [is red-faced (comment)]


Either way, 的 is acting as an adjective marker that marks 红红 as an adjective. and the 是 (is) between the topic and the comment is omitted
As for the rest of the explanation, I cannot explain it better than blackgreen did, so I am not going to try.
Different example:

[關羽的脸 (topic)][(是)红红的 (comment)] = [Guan Yu's face (topic)][(is) very red (comment)]


[關羽 (topic)][脸红红的 (comment)] = [Guan Yu (topic)][(is) red-faced (comment)]

Example of using 红红的 in SVO sentences:

[關羽][有] [一張红红的脸]
[Guan Yu (S)] [has (V)] [a very red face (O)]