
Hi Chrissy,
Unfortunately this is a common problem with having a young, immature and not yet fully trained dog when bringing in a puppy - they are
both puppies and are almost certainly going to find interacting with each other far more exciting and rewarding than doing so with you unless you ensure that both get lots of separate time, attention and training with you :) It is not an easy task because the new puppy will obviously need lots of attention towards socialising and house training while you still have to develop a lot of basic good manners in your teenager

and find time / energy for him to spend quality periods with you that will ensure he retains the bond shared prior to the Young Pretender's arrival ;)
I'd doubt that your older boy is "marking his territory" as such - probably just getting over stimulated by the presence of your new addition.
regards, Teri