
The very hard poo will be emptying out his anal sacs (this is good), they may have been a bit full if you are noticing the smell. I imagine this is the first time he has had a bone, and doing such hard poos is a bit of a surprise.
I would also advise in future getting a marrow bone from the butfcher, letting him chew it raw for a few hours, and if you want it not to get smelly put it ion a roasting tray for 45 minutes on gas mark 9. this stops them getting wiffy, but they don't go as brittle as the commercial toast bones, which I supect must be done on a higher temp, probably in a kiln.
Also most butchers will only charge you 25 to 50p for a whole bone, sawn in half, some ask only for a donation in their charity box!
Mine have had their bones like this for 11 years, and the eldest has her lovely teeth commented on every time she has her jabs.