There's a Ranger(Level 4)/Scout(Level 3) in a campaign I'm in, and he tricked me into give him a tiger very very early because I hadn't even read the page yet or knew what I was agreeing to. He currently only has an effective Druid level of 2. My question about this is, how would that work according to the SRD? Is the tiger level 2, or, since there's a -6 Druid Level Modification on having a tiger as a pet, is it level 1 until he gets 16 levels in ranger?
First of all, I want to state that the “Tiger’s level” isn’t really a thing, or at least nothing relevant. There is the character’s “effective Druid level,” which determines the Animal Companion’s benefits per The Druid’s Animal Companion. These include bonus HD, which do wind up increasing the Animal Companion’s “level,” but that’s not really relevant to the question. Animal Companions do not simply “level up” with their masters; they just gain bonuses (including more HD) when their masters level.
How the rules work:
He counts as a Druid 2 for the purposes of his Animal Companion. A Druid 2 cannot have a Tiger; he must be at least Druid 7 so that he “counts as” a Druid 1 (after the −6 penalty) with respect to the Tiger.
As a Ranger, he would not count as an effective Druid 7 until Ranger 14, at which point he would get a regular Tiger and only the 1st-level benefits listed in the table, i.e. Link, Share Spells, and 1 Bonus Trick.
Personal opinion: this is dumb, and it should be the other way around
The Ranger gets an Animal Companion so far behind the Druid that it’s basically good for not much more than scouting. Meanwhile, the Druid gets an Animal Companion that can be nearly as good as the Fighter. This is stupid because the Druid without an Animal Companion is stronger than the Ranger and his own, better Animal Companion combined. The Druid is one of the most powerful classes in the game, but even though the Animal Companion is very good, it’s not even the primary reason (or even the secondary reason, for that matter) the Druid is so good (it would be #1 spellcasting, #2 Wild Shape, #3 Animal Companion).
So I usually houserule that Ranger’s get the better Animal Companion (starting at 1st and at a level equal to what a Druid usually gets, rather than halved), and the Druid gets the weaker version (if he gets an Animal Companion at all, if I can even be convinced to allow Druid; I don’t outright ban it but I try to help players who have character concepts based on Druid express that same concept, if possible, with another class).
What you might do in your game
Anyway, back to your Tiger. As a 7th-level character, this is what a single-classed Druid could get; I’d be inclined to just allow it for the sake of not upsetting things. I’d let the character use both Ranger and Scout levels for it, and I’d let him have a “full-level” Animal Companion, so a regular (effective Druid level 1) Tiger is fine for his character.
In a couple of levels, assuming he continues in Ranger or Scout (or something else I felt was appropriate for advancing it), the Tiger might start to get better (i.e. effective Druid level 3, he gets 2 bonus HD, +2 to his Natural Armor bonus, +1 to Str/Dex, another Bonus Trick, and Evasion, and then so on as he continues to level).
I might ask the player to take a feat for the benefit of using any class level, rather than just Ranger or Druid levels, to count for the purposes of his Animal Companion.
If you don’t have a Druid in the party (and it sounds like you don’t), these are pretty easy changes to make. If you do, you may have to let them both have the “strong” Animal Companion, since the Druid player probably doesn’t want to lose his pet. That’s probably fine: it’s not like you were going to fix the balance problems around the Druid with this change anyway.