Has someone been feeding him?
Change in routine?
Dramatic changes in “personality” can unfortunately often be ulcers too.

Ulcers was the first thing that occurred to me on reading this - unless there’s been a dramatic change in routine otherwise.
Is the grass coming through where you are? Did he have the same reaction last year when it started growing?

Yes could be ulcers, horses that are prone to them can have a flare up caused by only small things, how is your livery yard on making sure they always have forage as this is really important for ulcer prevention, I livery on a similar yard and have had to make myself 'that owner' on reminding them that in their livery agreements they commit to doing late night and early morning top-ups, ensuring they use trickle nets and so forth for the greedy ones - they'd never ever leave the horses without water but some of the grooms seem to think it's fine for them to wait hours for hay which is almost as important to their wellbeing IMO.

The other thing could be that something (could be relatively minor) has happened to make him feel threatened in his stable? Have his neighbours recently changed, or any new members of staff he may dislike? Could even have been the storms a few months ago which def unsettled some of ours. Again I think we do sometimes need to remind ourselves how unnatural it is for horses as a prey animal to be confined in a relatively small space (as humans we tend to think of their stable as being their cosy/safe area and some horses seem to feel that way too but it's not a given) - if they then feel 'invaded' or unsafe in there that can result in aggressive behaviours to anyone approaching them as a defensive mechanism - since 'flight' is not possible then they feel the need to 'fight'. I wonder if he's better if you take him outside the stable to groom him, if he's only showing these behaviours inside the stable? Or is it anytime he's being groomed/handled on the ground?

My first thought was ulcers - DPony changed in personality quite a bit before his diagnosis, became very grumpy and "off". Now a different pony after treatment.

Would agree with ulcers or something else causing pain. If he is ridden he may associate you being in with him as being tacked up which can be a pain trigger.

Ive put both of mine on marshmallow root, google it, fab stuff, cheap as chips and cheered my newly grumpy boy right up.

I'd be getting an equine dentist to do his teeth, and making sure that his saddle fits properly (get a saddle fitter out if you haven't already!), and getting his back checked along with thinking about possible ulcers. If being tacked up and ridden is making him sore that could be another reason that he's biting! If there's another reason he's sore (such as arthritis somewhere causing chronic pain) then that can also lead to ulcers - they're often caused by pain somewhere else. How much turnout time is he getting also? Also I'd make sure you're not feeding him treats and that no one else is! My friends mare gets very bitey when she is fed treats and has improved a lot since we banned them completely!