Hi there. I am no expert but my youngster was a prolonged mouther/ biter so I thought I'd offer some support as I know how you feel with a small furry shark in the house!
Did you say he has always been bitey but Is now worse? I wonder if it may be worth considering some ideas:
1)He was castrated and then only a week later dad leaves so he is probably feeling pretty weird. His hormones will be out of balance and also he's wondering where his buddy has gone, so any behaviours he already has may be exaggerated.
2) I believe if a dog has aggressive traits (not saying yours has) then castration is not necessarily a good idea.
3) I think there is a lot of new thinking on the whole alpha thing-many behaviourist now believe that dogs live more like families than heirarchys as previously thought.
My youngster is now nearly 15 months. She was a hellish biter and like you, I found it really difficult to train her as she would just go over the top and start jumping and nipping me. What I did (apart from drinking lots of wine) was just accept that she couldn't cope. 'Training' sessions were kept short and stopped as soon as she started being silly.
Every time she nipped I froze. I mean no movement. No eye contact, no talking, no touching. Nothing. If she carried on I would not move. If it got painful I would zip out of the room and shut the door behind me.
We did this for a very long time-not the old 'your puppy will soon get the idea'....that you read in all the dog training books! We also didn't find that 'telling her off' really worked (although we did periodically get cross
). Ours is a very bold tough dog. She doesn't scare easily, but she does love to be with us, so exclusion was the only thing that worked for us. We trained her to go and get something in her mouth for those moments where she found it really hard to control herself (first thing in the morning etc).
We gradually reduced what we would put up with and would calmly say 'no biting' and put her out of the room for a minute.-Eventually for any teeth on skin contact.
Ours had her first season at 12.5 months and I think she grew up a bit then, and the biting sessions gradually decreased over the next few weeks. We now have a dog where if she does have a rare moment of relapse we really notice it and we look at her in shock, at which point she remembers she's not doing that any more!
So, sorry for long post but I feel for you! and want to say just keep your cool
. Calmly take charge of the situation in a consistent way and I'm sure you will be out of the worst of it very soon.