Different dogs respond differently to the chocolate. It does take quite a bit of normal milk chocolate to cause the toxic reaction. Also, your 15kg lab pup would have ingested a LETHAL dose with 7 pounds - he will get sick or possibly die with a much lower dose. Also understand that the theobromine stays in the dog's body for several days. This means he does NOT have to eat it in one sitting - 7 pounds over a few days time will still be lethal. And since he doesn't need the full 7 pounds unless you want him to die for sure, 1-2 pounds over a half-week is probably enough to put him in the hospital.
So while an occasional candy bar won't hurt bowser, it's not good for him, and you won't want to do it regularly. I have a small dog that occasionally gets chocolate from my kids or granddaughter and it hasn't hurt her, but we don't give it to her as a regular treat either.