Riccardo, you will be surprised but both sentences sound correctly, both options are possible. The difference is that the first sentence Если его оставить... sounds impersonal, and expresses smth. in general. When you say "Если оставишь его..." sounds like you're addressing this message to a particular person.

It is much like the passive and active voice in English. "If he's left in a botanical garden" vs "If you leave him in a botanical garden".

I can't tell you precise rule, but they are both correct. The meaning is a bit different though. The first sentence (оставить) describes abstract situation, the general tendency of something. Literally, the second sentence implies that the person to whom this sentence is said is actually going to perform the action. It omits the subject (ты), but you can add it there: "Если ты его оставишь...". Although the second sentence could be used to tell general tendence too, but it has more realistic mood in it. You can use the second one when you are more sure because it has already happened before. I hope I didn't confuse matters even further.

Я, как носитель языка, восприняла бы первое предложение как информацию, а второе как небольшую жалобу. Ну нельзя оствить без присмотра, обязательно что-нибудь случиться. Вот опять открытие сделал)))