Eddy is a single father of an 8 years old son, Junior.

男の子は８歳だ　＝　"The boy is 8 years old"
"_ is x years old" = predicative

８歳である男の子　＝ "an 8 year old boy"
"x year old _" = attributive

He hides his leather pants and whip in his drawer, and this is whyso Junior thinks Eddy is a lion trainer.

Sometimes he comes up with unrealistic ideas that make other characters frown upon.

Eddy is a single father of an 8 years old son, Junior.

In this sentence, ”8 year old" is used as an adjectival phrase to describe Junior. That's why we don't use the plural form of "year" here.

He hides his leather pants and whip in his drawer, and so this is why Junior thinks Eddy is a lion trainer.

ちょっと変なストーリーですね。

Sometimes he comes up with unrealistic ideas that make other characters frown onupon him.

Eddy is a single father of an 8 years old sonboy, Junior.

It feels redundant to say he is a father of a son.

Eddy often seeks a dates in story lessons.

He hides his leather pants and whip in his drawer, and this is why Junior thinks Eddy is a lion trainer.

😮

Sometimes he comes up with unrealistic ideas that make other characters frown onside eye him.

Nuance:
side eye - they give him a bad look but don't say anything
frown upon - disapprove of, but don't necessarily actually make a facial expression

I'm not sure which one you meant so including as notes.