He sounds like he's doing the typical boundary pushing of a teenager - what ever you decide is the right way to handle things then do it, stick to it and dont give him an inch. Rotts by nature are bold, confident dogs (as I'm sure you know by now!!) so they do need firm but fair, consistant guidance.
Bet ya get loads of fun out of him though - I know I do with mine - I find Rotts to be such characterful dogs - I giggle at mine every single day
