"By that time, I will have gone to him."
Translation:Tegen die tijd, zal ik naar hem toe zijn gegaan.
In the absolute sense, naar is like 'to' [to · for · at · by · towards · after · up to · according to · toward · as per · according as] whereas naartoe is like 'there' [much more flexible]. The sentence doesn't really need naartoe, as badpinguin1986 pointed out above. Naartoe is actually used less for physical direction and more for philosophical direction, such as, "jullie kunnen niet weten waar jou geld naartoe gaat" or, "Wij vragen ons af of wij kunnen proberen oplossingen te vinden en daar naartoe te werken." That said, you will also see naartoe in physical direction sentences as well. I think, just from mulling this over, that we use naar when we have a specific place. "Wij gaan naar de park" but we use naartoe if we don't know, "Waar gaan je naartoe?" although I'm just as likely to say "Waar gaan je heen?" but that may be Flemish or it may not be.
Good question. I was using the relaxed term of je for jullie. In spoken Dutch, if there's a group of people you usually just say 'je' meaning 'jullie'. If I were just speaking to one person, then yes, it would be "Waar ga je heen/naartoe?" This is a good example of using the verb conjugation to figure out who the person is talking to. Another confusing person is they/she. I could ask "Waar gaan ze heen?" and you know I'm talking about 'they' and not 'she'. If I asked "Waar gaat ze heen?" then it's clear that I'm asking where 'she' is going, and not 'they'.