It is common behaviour in ANY breed of dog at this age. He is going through puberty and displaying the "normal" characteristics of a male dog. This usually includes, dominant (or aggressive behaviour), and increased sexual behaviour.
This needs to be rectified early as soon what starts as a hormonal problem becomes a learnt habit and is harder to solve.
If you are not going to breed from him, then castration is the best solution and can be done now. Don't listen to breeders who tell you "big dogs can't be done until they are fully grown". This is a myth.
If you don't want castration, there is a drug called Tardak (UK) which is a from of temporary chemical castration which lasts around 2-3 weeks. This can be repeated and will allow breeding later on. It is EXPENSIVE though so think carefully.
My best advice here is - if you are having a problem at 8 months, get it sorted. Back up castration with some behavioural training and then you should have a nice dog with no problems.
Good luck