Hello, congrats on the new pup.
The first thing you really need to do is get radiographs ASAP on those front legs to confirm or rule out a diagnosis. Yes this could be related to diet very easily but medical intervention will also be necessary to get him through this as comfortably as possible.
Radiographs by an orthopedic specialist is what I would recommend. Then proceed as needed. There are a few conditions that can cause this which include HOD (hypertrophic osteodystrophy), pano, and OCD (osteochondritis dessicans). Physical therapy should also be instituted as soon as possible.
Treatment for these conditions are usually supportive care, change in diet, and pain management. In some cases splinting or wrapping the legs for added support during recovery can also help to improve the condition and allow the pup to be as active as possible.
I would put him on a high quality diet for a large breed dog. Proplan should have large breed puppy as an option.
I also suggest keeping him off of hard surfaces or slick surfaces until he is at least 8 months old as much as possible.
Protein levels should range from 23-28% for the growing boy and don't restrict his exercise just be sure it is on carpet, rugs, or grass because his bones are still softer than normal and can be affected by this.
Is this painful for the puppy, it seems that the more he plays the worse it gets?