It could be! Every boxer is different and it all depends what he is used to. The fact that he is lagging could be a good indication that he is no longer enjoying himself. He could be developing arthritis in his joints and running could be painful for him. I would take him to the vet and have him checked out. A 3 mile walk might be a better choice.

If you really like a jogging companion, it might be time to think about your next boxer as they need to be at least 18 months old before you can take them jogging with you. Of course you could always rescue an older dog.
Please go and post your introduction in the Introduction Forum. This actually should have been your first post. It is something that you agreed to when you joined this forum!