Here the word "you" is plural and refers to the people to whom John wrote, as well as all believers. The word "he" is emphatic and refers to Christ. (See: [[rc://en/ta/man/jit/figs-you]])
"remember and believe what you have heard from the beginning." How they heard it, what they heard, and what "the beginning" means can be made explicit: Alternate translation: "continue to trust what we have taught you about Jesus just as you have trusted since you first became believers" (See: [[rc://en/ta/man/jit/figs-explicit]])
To "remain in" means to continue to have fellowship with. See how you translated a similar phrase to "remain in" in [1 John 2:6](../02/06.md). Alternate translation: "also continue to have fellowship with the Son and with the Father" or "also stay joined to the Son and to the Father" (See: [[rc://en/ta/man/jit/figs-metaphor]])