Grandma

Hi, I,m new at this so please be patient. I have just taken in a new puppy about a week ago. He was found on the interstate highway, sitting by his dead mother's body. We know nothing about him except his only purpose is to please. He has gotten a clean bill of health from the vet and seems quite content with his new home. He has learned how to sit, stay, over, and doing quite well with the "potty outside". We have tried to quess what mix he might be (picture in puppy pictures under grandma). We quess about 3-4 months in age and weighs 6.3 lbs. Possibly terrier, maybe chihuahua---any input would be welcomed. Does anyone have any experience with this mixture or any ideas what we might expect from this little guy????? Any advise would be welcome. We have had many dogs, but none so small. Just to give you an idea---our Sophie is 6 yrs. old and a Great Pyranees. So big with lots of long hair is what we know, not short hair and tiny. He is kind of like a wind-up toy to her, but very gentle. Thanks for any input.
Grandma from Ohio