[JoeyP]

This is a form of aggression and I had a dog many years ago who did just the exact same thing

What you need to do is engage the help of someone the dog does really know that well
Have him stay for 10 mins or so
and then have him get up to go

Have him get up slowly .... the second your pup gets up or growls you get in front of him and say NO!
Get in front of him close enough to where he will need to look up at you
You need to break his stare
and his concentration

When he calms down and no longer goes to get up you praise him
try and repeat this again and again over a few sessions
But youll need to be very forceful with your NO!