I am also new to Yorkie ownership. Our Chloe is 8 mo.old is 2.5 lbs. and she needs alot of exercise and walking. She runs around alot ! My sisters dog on the other hand really just lays around. ( and her dog and mine are sisters from the same litter) so I guess all dogs have their own personality regardless of breed.
As for being social ...
I take Chloe with me just about everywhere except church. She is very social with people and she rarely barks , she seems to enjoy people.
As far as other dogs , she will play with our other dog ( a 7-8 lb. hyper terrier mix) and my mothers BIG dog which ( as best we can tell) is a mixture of a chow and a black shepherd and maybe a wolf ? She is all black , looks like a hell hound ( she even has a black tongue) and when on her hind legs stands a good 6 feet. She LOVES Chloe , and Chloe loves her and will run to see her when we go to my moms.. BUT if any other dogs come near her ,she shakes and hides behind me... go figure a six foot hell hound she loves , another 3 lb. dog scares her
so I don't think she likes other dogs to well yet.
Your little guy went through some big changes , so maybe he just needs consistency for a bit , until he gets back to his social self.As for exercise I found the tunnels that little kids crawl through , my dogs love...she will occupy herself for an hour just running through it
