I also have a rescue from a puppymill bust, and we have had her since the end of December 2008. Lane has come a very long way too, and it has been an amazing transformation to watch. You are definitely beating us on the potty training though! Lane thinks a pee pad means you squat about 3 inches in front of it (preferably on the carpet).
She also loves to do the little quarter-sized marking spots much like a boy, so we have just decided to keep little undies on her so when she marks she isn't decorating our carpet.
I think the major difference in our situation is that we already had another yorkie at home, and she has taken to her completely. Rory is a happy, go-lucky girl, but she was a bit of a spoiled princess before Lane came along. I can't tell you how sweet Rory was/continues to be towards Lane. Lane first modeled everything she did after Rory and continues to do so. Rory's favorite game has always been fetch, and wouldn't you know it Lane is now obsessed with fetch too. Rory climbs in Daddy's lap, and Lane wants to be in his lap too. Rory runs to the other room...guess who follows no matter what is going on where she currently is. I think dogs from a mill will most likely always have a closer bond with other dogs because that is all they knew before they were rescued. I hope you can continue to take him to play with Bernie and maybe find some dog parks or a doggie daycare in your area. Lane was a bit timid there at first, but doggie daycare is Rory's favorite place on earth and Lane quickly adjusted there too.
Every now and then when they are having a hard play session and Lane gets really fired up, she'll pick up any toy on the floor and shake it (as she's watched Rory do). The funny thing is she shakes her head in a wider motion so it doesn't quite look "normal," but we are so happy to see her playing since when we got her she had no idea what a toy was. She'll also occasionally grab a dog bed and try to shake it too. I'm not sure if your Sammie gets zoomies, but it is the best feeling in the world to see them running free knowing they'd been caged up (to put it mildly for the first part of their life).
I'm big on clicker-training, and Rory knows a bunch of tricks and catches on very quickly. We tried and tried with Lane and within the last 6 months she has really started to enjoy training time, and I can see her confidence grow as she starts to figure things out. I know you said he wasn't food motivated (Rory is...so maybe that is why Lane is...she also sings for her supper now too), but if you can think of any high-valued item he might be excited about, he might really enjoy some training too.
Just the other morning, Rory was extra sleepy and stayed in bed after I got up. Lane followed me out into the kitchen and I was sitting with her on the floor loving on her and petting her and she still kept looking over her shoulder to see where Rory was. I got out one of her toys and she didn't care at all...Rory finally woke up and Lane was all about that toy once Rory wanted to play with it.
Sorry that was soooo long. I have loved watching the transformation of my girl, and I know you wish Sammie wanted to play at home the same way he does with Bernie. Maybe socialize him around lots of different dogs and see how he does with all of them. If he does well, maybe in the future you could consider adding another pup for him too.