at 7 months you could start getting him used to a harness and having things drag behind him like a small bike tire or milk jug filled with rocks or something. I wouldnt add much weight until closer to 18 months you dont want to over do it and injur his joints and such. I start light runs closer to a year mark 10 months or later but only start with maybe 10-15 minutes light running and gradually work up until they can go out for a full run with me. At this age its a perfect time for working all his basic obedience work on stay , recalls, sit downs ect . If you want to do weight pull its improtant they learn to focus on you as shows can be very distracting with other dogs and people and noise so working on recalls is important. Work him more mentally right now If I were you.
Of course walks and games of fetch are always good forms of work outs at this age and any age.