Oh sorry to add. I just started with weights about a month ago he was just over 2 (due to old school mentality) and kick myself now!!!!
My boy weight 90lbs and I started with 5 he was like nah more please. Than upped it to 10 he was like cool. Than after 30-40 minutes he was exhausted. That was my desire. I need to keep walks around 30 minutes. So today I'm going to add 2lbs and just leave it there for awhile.
But I use a tactical vest. (Former Army Ranger). And I loved the dogs vests the canine bomb and MP handlers used. But what's cool about the "extra" 2lbs I'm going to add will be from water bottles, that way he will have quick access if in need!!!
Speaking of safety if you add weights keep them over his four corners close to the legs as possible. Or go the pulling harness route. I'm now in a wheelchair and tried to get him to pull me. Lol it didn't go so well so I went with the tactical vest. It looks silly to some being on a bully instead of a typical shepherd but whatever. Haters are haters.