may sound daft is it wormed upto date??
Also what have you been feeding it on upto now??....
Does it have diarrea?
If you've been swapping and changing the food, this can have an adverse efect of the dog.
One of my pups was very light, until I started to uup the raw meat........ within a couple of she'd wanged loads of weight on..and looked a completlely different dog...
Dogs can also been allergic to ceral in dry foods....I found this out recently to...
I think you need to tell us a bit more if you can
ps....couldnt see a picture, so Im assuming its a skinny un