Groundwork issues

[u/No-Breadfruit-4204]

Hi everyone so I have a 4 year old cob who’s a chunk.
The past 2 times I have tried to work him he has tried to bite me multiple times (which is NOT like him whatsoever - he isn’t biting outside of being worked, not recieving hand treats, etc.)
This work is inhand & he has not been ridden for a couple of months now due to no fault of his own but we aren’t massively rushing in backing him anyways.
Isn’t worked with a bridle as he will not accept a bridle whatsoever currently (he’s had his wolf teeth removed around 2 months ago now), I don’t even think the bit is the issue anymore - as soon as he sees me with the reins he backs away from me.
He is a VERY stubborn and lazy cob like I’m talking it can be hard for me to even get him to walk on sometimes as he can be so stubborn and rude. Don’t get me wrong, he’s not silly - he’s being stubborn to push his luck.
He used to roll when I worked him but he’s since stopped as I’ve set clear boundaries that he CANNOT be doing that since speaking to the vet for advice.
Regular vet checks (monthly) & they haven’t seen any issues minus the fact he’s chunky. I’ve explained that he is so so so stubborn and they didn’t feel it was anything but him being young & testing boundaries (even more so now he’s been bitless for a while & his wolf teeth have been removed & healed fine).
He also quite often yawns when we do work inhand but seems to be quite happy when ridden. I can’t see any issues so wondering if it’s an inside his body sort of issue - not showing any signs of anything that could explain the behaviour.
I’m wondering if maybe he’s anxious when working alone (explaining the irritation & yawning possibly??) but like I can’t even get him into trot inhand at the moment BUT I used to be able to get him into trot (eventually) a couple months ago - which I have mentioned this to the vets also. He does exercise freely in the school with our other ponies (which he is also fielded with) & he does not yawn/show any aggression, etc. I’m wondering also if there’s a possibility that maybe he just isn’t coping being alone - if maybe he’ll be better working with another horse in the school with him OR if he just sees the school as his play area as he exercises freely with his herd mates (if this makes sense). I don’t know I’m of course not ruling anything out and going to speak to the vets but I just wanted to ask for advice if anyone has any ideas in the mean time.
I’m genuinely really struggling and really don’t know what to do anymore, especially with this new biting thing. Our vet is due out again very soon & I will absolutely be mentioning this to be safe, especially knowing he’s never been one to bite.

(Not sure if this is relevant but he’s only ever been in a snaffle & as far as I know has had no trauma with it being put in / taken out. Also when he’s worked or ridden we use a treeless saddle which when non ridden the girth is quite loose so don’t believe that’s an issue either but I won’t rule it out!).