Why is he refusing? Is it a question of not wanting to part with the aircraft or is he unhappy with the price?
Joint ownership of an aircraft involves all of the owners having an interest in the proceeds of sale plus any profit derived from the operation. The trustee holds the aircraft on behalf of the beneficiaries of the trust (ie the co-owners). A trustee is under a legal duty to act in the best interests of the other owners and any owner can require the sale of the property.
The trustee cannot unreasonably refuse permission for the aircraft to be sold and you could obtain a court order removing him/her as trustee or requiring him to allow the sale to go ahead. Although there would be an expense (possibly quite a lot) to taking the trustee to court he is likely to be liable for the costs.
If he is not amenable to discussion, your best bet would be for the other owners to write to him threatening legal action and pointing out that he will be liable for the costs of such action. Better still would be to get a solicitor to write although this will cost you a few quid.