Its sounds very similar to what Barney does but poor Max, my golden ret, gets the brunt of it.
Like you, I was very alarmed and he started almost as soon as we brought him home. If he was out on the drive with the other two when a visitor arrived, they'd all naturally go and greet the visitor. But then Barney would start with growling and snapping and attaching himself to the back of Max's back legs. He'd do it until the visitor had come inside. Its sounds dreadful.
Like he's savaging Max. Apart from anything else its so embarrasing.
In the end my trainer friend came to witness him in action. She said the same as your trainer, he's just really excited. She pointed out that Max is paying absolutely no attention to him what so ever and we should take his lead. So I do, I ignore him. Occassionally I do step in and get him to sit and stay, which calms him.
It did start to ease but recently it ramped up again just as all his hormones were kicking in and other behaviour materialised.
So it does sound similar to me. Have you taught him to be sent to his bed/crate? I do that sometimes if they get mega excited when a visitor comes, and it really calms them down.
Basically you don't want to reward it in any way, ignoring him is one way. If its not working I think perhaps that if he does it at home the second he starts, walk out of the room and leave him there for 30 seconds then go back in and carry on as before. Repeat every time. This will teach him that behaviour makes you go away and he won't want that, so hopefully he'll stop. It is normal puppy behaviour, it doesn't mean he's going to be aggressive so don't worry about that.
