|Re: Help with personal pronouns please|
Why is it seinem Neffen? Because that's what it is.
However, you needn't panic (much), because there is a logical reason for it.
Neffe is one of those nouns whose declination descends ultimately from the old Germanic "weak declension". The nominative masculine singular has the basic form of the word, but all the other singular cases, and all the plural cases add the ending -n or -en.
From what I've observed, many nouns that denote a person's position or occupation follow this pattern, and in many cases the nouns, unlike Neffe are obvious loanwords from some other language, like der Regent and der Prinz. More familiar to us today might be der Student (although this pattern doesn't hold for his Professor).
This weak declension is evident only in the masculine forms of these words. Die Regentin, die Studentin, die Prinzessin, etc. follow the usual pattern for feminized nouns--they are uninflected in the singular, and add -nen in all four cases of the plural.
Unfortunately there's no hard and fast rule to give you the right answer every time, but it should at least help you to know what's going on.