Oh Digby that does sound tough for you.
At his age I would focus on having fun together. A good bond is priceless for training so just focus on getting that in place.
Only ever use his name for nice things - like calling him for treats. Don't use it to tell him off. Don't use it to call him over for something he won't like (use literally any other words for that or just go and get him). What you are trying to do is make a really strong link in his mind between hearing his name and something lovely happening. That way, in the future, he won't be able to resist coming over to see what lovely thing is going to happen this time. At his age it won't take very long at all before he makes that link. All you need to do is keep using his name for nice things until he is much, much older.
The biting DOES get better, I promise . If he bites you then he gets no attention at all. If he cannot leave you alone, despite ignoring him (some dogs seem to take it as a challenge) then pick him up and put him somewhere away from you for a minute or so. Baby gates are really useful for this because he can go the other side of the gate.
Walking on the leash takes dogs some time to get used to. In some ways you might find it easier to get rid of the leash altogether. If you have a large field or open space near you then fill your pocket full of treats, head to the middle of the field and be brave. Unclip the lead. The run off. At his age he will not let you leave him and will follow you. Just keep giving him a treat when he catches up. Make it fun and you will be instilling really good foundations for when he is older. Don't feel you have to walk him very far on the lead. At his age it is all about the experience of being outdoors.
Daily schedules are important. Pick a routine and stick to it. The puppy will fall in line. My day is not exactly the same every day but there are key points the dog can rely on knowing what to do:
- no one gets up until the alarm goes off, regardless of how awake he feels
- a walk first thing
- left alone in the kitchen to eat his breakfast
- if I am at my desk then I am not doing anything exciting so he settles down
- a walk mid afternoon
- nothing exciting happens in the evening so he has learned to play quietly
We train and play and go out in between, as I fancy, but these facts remain true for him.
All that said, the one thing that stood out for me most clearly when he was younger is that I spent too much time worrying "is this behaviour permanent?". It's easy to think the biting or the rock chewing or the lead walking will never get better. It will. It will all get better. I promise you that. Be patient and consistent. Every time you take a rock off him it will look like he is not learning anything but he is. Suddenly, one day you'll realise it's been a few days since he last ate a rock. Then, before you know it, it's just one of those things he did when he was young but grew out of.
I am not sure what other support you have but if you can find someone to puppy sit for a few hours that will also help you feel sane and help the puppy get used to being without you.
It gets better xx