An antibiotic should be investigated. If your vet is unwilling to prescribe an antibiotic, x-rays of the rabbit's head will likely show swelling in the inner ear region, a sign of infection. The dewormer is not going to treat head tilt as the tilt is most likely caused by an inner ear infection.
Please see these up-to-date references
http://www.medirabbit.com/EN/Neurology/head_tilt.htm
http://www.medirabbit.com/EN/Neurology/Otit/otitis.htm
The second link in particular is helpful because it will help you distinguish between EC and an inner ear infection based on involuntary movements of the eye, called nystagmus. Vertical nystagmus (pupils moving up and down rapidly) is a sign of EC. Horizontal nystagmus (pupils moving toward the nose then toward the tail) is a sign of ear infection.
My vet successfully and routinely doses antibiotics for animals much smaller than 1.5 lb. For instance, my hamster of approx 50g is on an antibiotic.
Dosing info for Bicillin is found here:
http://www.medirabbit.com/Safe_medication/Antibiotics/Safe_antibiotics.htm
For a rabbit of 1.5lb, the appropriate dose of Bicillin would be 32,000-57,000 units every other day. The drug usually comes in a solution containing 300,000 units/mL. It should be diluted at least 1:1 with a sterile saline for injection. Thus, the dose would be 0.1 to 0.2mL Bicillin plus an equal or greater volume of sterile saline. You would draw up 0.1 to 0.2mL of the Bicillin solution into a syringe, then draw up at least 0.1 to 0.2mL of the sterile diluent into the same syringe. Mix, tap out the bubbles, and inject sub-Q.
