Hi and welcome to COL
Having an anxious dog is not easy. One that’s just hit the teenage phase can be a nightmare and your boy is right at that phase! Here speaks the voice of experience....Pearl was an anxious dog. It took me a long time (probably 5 years) to really work out what was going on and why.
There are lots of things missing from your note that we need before anyone can really try to help on the forum. When he bit you, what was the trigger? Did he have some form of contraband , did he provide warning - low slow tail wag, staring at you, lip curl, grumble, full on growl, attempted snap - before biting?
Your behaviourist should have explored all this with you already. I am truly surprised that they are recommending drugs for such a young dog. He is, in effect, the equivalent of a 2.5 year old toddler and is likely having the same sort of tantrums that you see in supermarket aisles....before covid obviously
he will be testing boundaries and seeing how well he’s trained you at the moment!
Not only is he at that norty teenage phase but there are many other factors to consider too:
Cockers can go through a fearful phase and if your boy, as my girl was, is fear reactive it will manifest in the behaviour you have seen - though this may not be the case with your dog, we (COL) would need to see him to evaluate
Your boy is experiencing hormonal changes. Think human teenager with the brain and experience of a two year old - it’s no wonder he’s a bit confused!
Teething. Just when you think it’s over, out pop the wisdom teeth. It’s worth checking his mouth to make sure he isn’t in pain.
Then we have hearing changes. Dogs hearing (I’m told) doesn’t fully develop until 10 months old. What he has been used to for 8 months is starting to sound different and if he’s a confused, fearful, anxious boy that must be bewildering.
Another thought is if he is in pain or has an underlying condition. Has he been checked recently by the vet?
My best advice is to slow everything down for him. Go right back to basics with training. Lots of sit practice with positive praise (excited sounds from you, hands on affection and treats if you do) when he gets it right. On lead and heel walking, lots of practice, again lots of praise when he gets it right. Help him to understand what you want him to do. However obedient he appears to be right now he has at least a year of consistent, persistent, training before anyone of us could claim we have a fully trained dog.
What sort of routine do you have with him? Here, we have dogs that go out in to the garden for a run round in the morning, fed then ask to go back to their kennel (indoor kennels) they sleep for 2-3 hours. Out at lunchtime for a run round and short training session then a slow afternoon while I’m at work or my husband is pottering in the shed. Again, short blast round the garden (we do have an acre so it’s the equivalent of a walk) and some training, probably placeboards before settling down in an evening. We have four cockers age 11 to 2 years. My anxious girl died at the end of January but her routine became similar to the others. I have posted quite a bit on here over the years and detailed her progress (Pearl) and that of Coral. Your boy should be sleeping around 18 hours a day, any less and he is likely to be fractious just as a toddler would be.
Hope the above helps. If you let us know which part of the country/world you are in, we may be able to recommend an ABPT Behaviourist or COL help locally.
Oh and you may want to stock up on wine and chocolate, it’s sort of a given with the ownership of a cocker
Top advice from Bizzylizzy re the Mime book. I will ask Archiebean to pop on to the forum as she has a resource guarded and can provide good advice and help.
It does get better, honestly - well done for reaching out on here,
Jayne