My new dog is afraid of men

Question
I recently adopted a 2-3 year old chihauhaua/beagle mix.  He is great with women and children, and fine with men outside of his house (ie dog park or on walks).  However, if a man comes into or up to our house he barks furiously and displays fear behavior (tail and head tucked).  Last night he nipped at my boyfriend as he was walking away.  I love this dog and he is otherwise perfect.  Can you give me some tips on how to train this dog out of this fearful behavior please?

Thanks!!

Answer
Probably the best way to correct this is with a trainer that can come to our house so he/she can see how the dog is acting whena  man comes to your house,and heshe can show you hoiw to correct this,as it is a serious problem and only a trainer can fix this-as this type of problem is better solved on a dog to dog basis,as some dogs are more fearful,others aggressive and some in the middle