am really surprised your trainer suggested this, poor Mostyn
If this (and the pack theory) is the method of training they are promoting then I would seriously think about changing puppy classes.
By scruffing him and frightening him like this you are enhancing his fear, and of course he will respond by trying to bite you out of sheer fright - that's the only way he knows how to communicate. I agree fully with Mari's post, and her suggestion of time out, and the 'OWW!!' is a much better idea. It sounds like the bathroom is just making him worse, for a pup to defaecate in fear is really sad
Does he have a crate or am area that he can be put in (that is not scary) for a time out?
He is really still such a baby, and hasn't learnt about bite inhibition properly yet - I wouldn't call this aggressive behaviour, just snappy turtle puppy behaviour, and yes it can really hurt! If you do think it's more than that please contact a behaviourist who will be able to see how and why Mostyn reacts - it will really help your relationship with Mostyn