It’s It's been solved! It’s The Shadow ManIt's The Shadow Man (1986) by StephanStephen Gresham.
From Goodreads:
Joey felt a chill as he stared at the luminous green dots on the screen of his ComputerAssisted*Playmate. What did CAP know about The Shadow Man? Joey's dad said there was nothing to be scared of because The Shadow Man wasn't real. But CAP, Joey's best friend in the world, would never lie to him. Or let anything bad happen to him.
But even CAP can't help Joey when The Shadow Man comes to visit him. The Shadow Man can hide anywhere. Under the bed. In the closet. Behind the mirror. The Shadow Man can even make Joey do things a little boy should never do. Especially when it hides in the sophisticated circuitry of the one thing Joey trusts most...