Scared Chihuahua

Question
My wife works at an animal shelter and recently we moved and decided to foster a 5 year old Chihuahua.
She is very attached to the pup and he is to her. The first few days at home pup was a bit scared but still sociable, allowing me to pet him, carry him and accepted treats from me. He is house broken for the most part, he appears to be leash trained and kennel trained. After the first few days he started snapping at me. Sitting in my wifes' lap he would snap and try to bite. He also tried a few times when not in her lap. Both of us took part in disciplining him, loud "no's and put in the kennel for a bit.
more recently pup seems terrified of me while wife is working. Will not accept treats. Will not leave kennel without being dragged and refuses any type of affection. As soon as wife gets home he is ecstatic, will sniff and even lick my hand, accepts treats, will even let me pet him a bit. But as soon as wife is gone for day he cowers, hides and is back to one seriously scared pup.
Anything I can do? We really want to keep the pup. I can accept the behavior. Just wonder why it changed and if there is anything I can do to get him to accept me and not be scared to death  

Answer
Kevin;
When you "Rescue" a dog...you never know what happened to it before you got involved....thus, you are experiencing the "Trauma" of the relocation, and the new equation set before him!
You will have to be very patient..this dog is grown, and he will need to feel really secure before he accepts what is going on in his new home. It is obvious you are trying, and that is great, but, be aware, that he may need to be just bonded to one person, and never accept you!
That is just the luck of the dice when you "Rescue" one.
Also, some mini breeds are more this way...
So, this being said, just keep your movements calm.
"No" does not get the dog to understand what is required, so, instead,
Give the dog a command, like "Stop". And if he does not mind, do give him a "Time Out" like you said before in his Crate!
We  have a new book out: "And Puppy Makes 3, or 4, or 5, or 6", which would give you an ongoing help and tool. It is available for $5.99 as and E Book at our Publisher's site: www.heartsonfirebooks.com
Check it out, I think there would be a good amount of tips and help to use throughout the life of your new dog!
Let me know what you think, and  best wishes to you and your family!
Regards,
Melissa