Dog suddenly scared of one family member

Question:
 My girlfriend and her mother recently adopted a 7 year old, un-neutered collie cross from a rescue centre named Walter. Not much is known of Walter's life before December, when he was found on the streets emaciated and ill. He has always had a lovely nature, apparently, was a favourite amongst the carers at the home, and the only reason he hadnt found a new family by now is because of his age. But we loved him instantly and took him home. I'm 20, my girlfriend is 18, and her and her mother are experienced dog owners. The 3 of us are really devoted to him and make sure he is almost never left alone, looking after him in shifts etc. We've had him for about 4 weeks and for that the first 2 weeks time he was AMAZING. The perfect dog- we couldnt believe our luck. He was well trained, had no accidents, has no interest in other dogs and doesnt bark at them or pull toward them. Because of his poor health beforehand, he had been on puppy food for the 2 or 3 weeks but we gradually introduced him to adult dog food. Since just about the same time, he has had a complete change of attitude. He now seems terrified of my girlfriend. She is very calm with him, not loud or high pitched, not too firm but firm enough when he is naughty etc. But whenever she goes to stroke him now he emits a low growl. He doesnt act aggresivley, he'll just sit there, hang his head as though in shame, and growl. Doesnt expose the teeth, but will just make the growl for as long as she strokes him, and if he can, will get up and move off elsewhere. He wont respond to her call, and wont go anywhere near her when walking around the house. But Walter loves being taken for walks, and as soon as my girlfriend picks up the leash she is his best friend again, but as soon as the walk is over he reverts back to his ways. He'll even think twice about eating food that she puts out for him. Yesterday my girlfriend called on him to come see her, and he just went behind her mother and hid. When her mother instructed him to go see my girlfriend, he walked over but he was seriously shaking like a leaf. Also, curiously, he has become increasingly attatched to her mother. He actually wont leave her side. If she shifts on the couch he'll jump to look at her to make sure she isnt leaving the room and sleeps outside her bedroom desperate for her door to open. Whenever i visit he is great with me, playing with me etc. He is fine with strangers and new places, and even loves our 6 year old neice (maybe a little too much- slight humpage). So its pretty heartbreaking for us to see him refuse to bond with someone who loves him more than anyone else. We thought he was testing his dominance at first, so we made my girlfriend the primary carer and do everything for him so he completely relied on her for walks and feeding, and the rest of us ignored him for a while but it hasnt had any effect. No amount of telling off or affection seems to change his attitude. The only change in his care has been the switch to adult dog food, and getting it served to him twice daily instead of the 3 times daily bowls of puppy food he was getting to help him put weight back on. Anyone have any clue why he has started acting like this? Or any kind of remedy? He showed no sign of this for the first 2 weeks so we are completely at a loss. Thanks, Dave
Answer:
 Sorry about the length. Didnt realise paragraphing doesnt work here. Cal me a newb. Sorry again. Anyone who reads that whole thing is amazing.
Answer:
 In the US NO rescue would EVER adopt out an uneutered dog. Getting him neutered may help some of the prolem & will go a long way toward preventing cancer. If this problem seems to have started with a change in food, then change again. I had some undesirable behavior with my 2 when they were on a higher protien food so can agree that that may be part of the cause. Part of this you all have caused by not leaving this dog alone. He now has some separation anxiety issues which will only get worse. IMO the dog will fare much better if you all get on with your lives in a normal manner than trying to cater to his every whim.