If you're really not sure or comfortable with the amount of calcium your dog is getting, you can always crush up the eggshell of your boiled egg and mix it in with green tripe. That should solve your problem.
Also, the gnawing does afford for some calcium. Maybe his jaw simply isn't strong enough to work the bones you are giving him, but at least he's gnawing , and that works the gum line, cleans tartar, and strenghtens jaw muscles ( everything good about that), and if he's eating chicken several times a week, he's getting his bone ration anyways.
Bones whether pork or beef and essentially definied by their size and purpose. Weight bearing bones, such as the tibia and fibula are going to be far more dense and strong, simply because of the task set before them. Ribs are easier, softer and more cartilege like, hence easier to consume.. but let's face it... the tibia of a mouse is going to be easier to consume, than the tibia of a giraffe, simply due to size alone, so common sense will tell you that if you have a boston terrier, realistically don't expect him to be able to masticate the same bone that a huge mouthed mastiff could manage. You have a doxy, they have small, little mouths and he's still a young guy.