This is a tricky thing, and is dependent on a grammatical construction that has limitations. The rule here can be called the rule of the nearest antecedent. There are two antecedents in this phrase both governed by the preposition εν in Greek, a preposition that roughly corresponds to our "in" [The NWT is particularly deceptive here in that in degrades the word "in" by two distinctive usages, neither of which is contextual:
"W know.....in the One who is True [the Father] ......in His Son Jesus Christ". He is the True God and life everlasting"
The two antecedents are
1.The Father
2The Son.
Then John uses the singular pronoun "He" [ ὁυτος the masculine form of "this", hence "he"]. Who is this HE? The Father or the Son, Jesus Christ?
When such a degree of ambiguity occurs the simplest thing to do is to go to the nearest antecedent and conclude that John was applying the "He" to the Son. This is the case in point in most occurrences of this construction. BUT NOT ALWAYS, and that is the catch. There are many occasions when NT writers apply the first antecedent to the pronoun, rather than the nearest.
The point is: can the Watchtower deny the deity of Christ here? No. Are you right in concluding that John is calling the Son God here? Yes. Let no one take that away from you.
And whereas I agree with you about the deity of Christ here, I cannot deny that others have the right to conclude otherwise. To me, I am told to go to the Son Jesus Christ for life everlasting [Jo 3:16, 36] so I associate "everlasting life" with the Son primarily, so I have no problem with concluding that the "He" applies to Him.
Also, remember, that what is said of the Father is said of the Son. We are IN the Father, and we are IN the Son, so accepting the deity of Christ here is not denying the deity of the Father, and accepting the deity of the Father here is not denying the deity of the Son either.
There are many evangelical theologians, such as Murray Harris who believe that the "HE" here refers to the Father, but this does not define his relationship with the Son as anything less than the Son being God.