If you have any doubts about the puppy's intentions, then yes, get a behaviourist, but otherwise, normal puppy classes should suffice. It sounds to me like normal, over exuberant behaviour, and growling is not always a warning, it can mean other things. He may simply be getting excited at the prospect of a walk.
That said, any puppy should be well past the mouthing stage at 8 months old, and I would make efforts to stop that ASAP. This article gives extremely good advice on how to deal with biting/mouthing in puppies and works, but you have to follow it exactly, and do the same every single time the puppy mouths.
www.cockersonline.co.uk/discuss/index.php?topic=64170.0;wap2
I would also follow something like 'NILIF' (nothing in life is free) which basically means in order to get or gain anything, the dog has to do something to earn it. A lot of dog owners do this anyway without thinking, but it basically means, to get his dinner, the dog has to sit nicely and wait to be told 'ok', to have his collar put on, the dog has to sit nicely and not growl/bite, to get at reat the dog has to do something like sit, down, roll over. You could also limit attention to times only when you request it, not on his demand. Some people think that allowing a dog to demand attention leads to dominance, I don't, but I do think it's a nuisance to have a dog constantly demanding attention.