Weight bags are so popular in abroad but in india, many owners use water bottles instead of weight bags, take two water bottles filled with water and tie two bottles in nylon rope ends (such that the end of the rope tied with water bottle horizontaly or vertically). Place the rope on your dogs back such that one bottle in his right side and another in his left side and take him for walk. I tried similar thing for Bruno but his main focus on the water bottle not on the walk, so i dropped the plan. But I see my friend boxer doing well with this kind of exercise.
I personally dont prefer weight bag exercise for 6 months old puppy as he is too young and these exercise i feel harm them. You can start such kind of exercise once he is 18 months old, as of now you can give him exercise only in the form of play.
For labs more exercise needed mentally than physically ( i feel so and i applied on my lab and it works well). What I did when my lab is hyperactive on his six months old is, I put some chicken pieces (somewhat bigger) on kong and give to him, and he assume that his task is to take the chicken piece out, he takes out in 20 min max and eat that (Note that you need to put treats in kong only that your dog is craze to eat). After that if i feel still he is hyperactive, i play hide and seek exercise with him, I hide the toy in any of room in my house and his task to find out the toy and this play go for about 20~30 minutes. Fetching the ball is another awesome exercise which make them tired in atleast 20 runs, after all i too give him mutton hoofs (as i dont prefer commercial dog bones which available in pet shops and most of the bones made up of beef marrow) when he is hyper, chewing bones also lose their energy.