@Frosty6611 I work at a lettings agents, and this is how we understand your kind of situation.
If he registers your deposit now, it doesn't matter, it's still too late for him and you can potentially make a claim.
If he doesn't have an inventory, he can't prove anything is in a different state.
He is basically up Shit Creek.
You are the one who holds all the cards here.
He wants you to sign another tenancy, at which point he will try and do an inventory and register the deposit. Presumably, if wants to take an inventory, he needs to come and do one NOW as he doesn't have retrospective data. So anything in the last 6 years is irrelevant, as he can't prove it.
However, I'm going to go out on a limb and say he's trying to justify keeping your deposit. I would suggest that you have a frank discussion with him- tell him you aren't interested in going on to a new AST and are happy with your rolling contract, that you will happily give 1 month's notice, and if you think that you have damaged something that you will pay for, then agree a cost now (NOT wear and tear damage after 6 years!). Make it clear that apart from this, you expect all of your deposit back.
If he tries to argue about this, tell him that you are aware of the repercussions of him not insuring your deposit, but you are not planning on taking any action unless he tries to hold on to it unnecessarily. In that case, you are prepared to take him to court for it, where he will have to pay the 1-3 plus both of your costs.
If he still contests, then ask him to go post the situation from his perspective on landlord zone and get back to you. I can almost guarantee he will come back and agree!
But don't withold your rent- don't give him reason to be a dickhead!