I will try to explain it using an analogy to Russian, as during my Russian course I've seen such an explanation many times, although I would never think about it myself. You probably know that "his" is jego in Polish, and его in Russian, "her" is jej/её, "their" is ich/их. We have "niego" instead of "jego" here. I've never thought about them even as similar words, but what I learned in Russian course is as follows: if the pronoun is preceded by a preposition, you add "н" at its beginning, thus making него, неё, них. And it clearly works similarly in Polish: 'jego' changes into 'od niego', 'jej' into 'do niej', 'ich' into 'dla nich' - whenever you have a preposition before. Seems like quite a good way to remember it.
That's definitely 'from'. As 'for' is concerned... I guess there's some situation when Polish would use 'od' and English would use 'for', but it doesn't come to my mind.
I am getting clothes for him - well, that would be 'dla niego', but I don't know about the verb, it depends what exactly would 'getting' here mean. Collecting them from your house? Buying them? Actually 'getting them' from some 3rd person?