If he's on solid food, it's not too early too start. I started Gage on full BARF at about 9-10 weeks, when we realized he was allergic to all grains.
One important thing - don't mix kibble and raw meat. Kibble takes longer to digest, so if you feed them together, the meat may stay in his digestive tract longer than it normally would, which makes him more vulnerable to food poisoning (salmonella, etc.). Instead, have two of his meals be kibble, and the other be BARF.
Chicken necks or chicken wings are a great place to start. It's recommended to start with just chicken, until he's fully on BARF, then slowly add things in. Easier on their tummies I guess. Personally, I went pretty much full tilt right away - although I didn't add many different veggies at first. Make sure when you feed the necks and wings that they are fully thawed, and, depending on your pup, you may need to hold onto one end, forcing him to chew so he doesn't swallow it whole and choke, or get a tummy ache. Gage would swallow whole necks, and then I could feel them in his stomach - it was gross and pretty freaky. Some pups aren't too sure about chewing, you may need to grind things up at first.
Also, an easy place to make a mistake is that most BARF websites just tell you to feed the dog 2% of it's body weight - puppies need up to 10% of their body weight. With a 7 week old pup, I would start at 8-10 percent of his body weight (once he's on full BARF, when it's only one meal, I'd feed a third of that, when it's two meals, two thirds of that, etc.), pay attention to his weight and how he looks, and adjust from there. It's important to weigh him once a week, and adjust his food accordingly, as they grow so fast!
As for starting yogurt, depends on the pup. I'd give it a try, and as long as it doesn't upset his tummy - great! Also, you may wish to add a bit of grated ginger, it's great for digestion, and helps their breath smell nice
Hope that helps!!!