Hi Theodorus! Lovely to hear from you (although I of course wish Theodore wasnt having a problem) 💚
Hopefully this is nothing grave, “just” a bit of age-related arthritis, making him stiff and less agile. Elderly rabbits with joint problems often respond well to a low maintenance dose of meloxicam.
A more sinister cause for a balance issue would be ec. It can affect any part of the nervous system, even if it’s mostly thought of as a cause of head tilt. It can affect the spinal nerves to the hind legs.
Another possible cause for a balance issue is an ear infection. Lops are prone. Sometimes wax and debris collect in the outer ear canal and it can be so much that it puts pressure on the ear drum. You can feel (gently obviously) under his ear base for a lump, which would be a telltale sign. At that stage it can be resolved by using eardrops to soften the wax plug so it can be expressed. We cant see into the ear canals, a vet would have to do that with an otoscope.
Do you see him straining to pee? Any (sudden onset) incontinence issues? Is he moving around seemingly without undue effort? (Except for when he had this wobbly spell).
Rabbits can also get vascular strokes. Its not very well known (some vets still call ec “bunny stroke”, which is rather confusing) but vascular strokes in rabbits are probably under-diagnosed. Rabbits can probably get a TIA, a transient ischemic attack from natural causes (dogs can).