Thread: is this behaviour anything to be worried about

No wonder you two are roped together, it looks like ya going up the Himalayas
Cool looking muttlie
I would say he is still going through his galloping puppy stage, i'm not sure what age Mastiffs mature, but Rotties mature at 3, so can take a while to settle down into a fairly predicable state. You've still got plenty of time to sort him out but as mentioned before, it is so much easier to get on top of a potential problem as it develops in the first couple of occurrences, then fix it after it's become established.
My current goof bag was a rescue dog I got at 2 years old and man did he have some annoying habits