Most dogs seem to forget everything when they hit their teens - I know mine certainly did
Zipper was a nightmare from 8 - 18 months but is tons better now.
Training becomes 1 step forward 2 steps back for a while - but keep persevering it will get better - 2 steps forward 1 step back
I would get a long line and use that for a few months, he won't have any choice but to obey you then -
That's what I did and now mine are 95% ok - food and flying birds are my downfall, but I'm working on it.