Whether a member of BALPA or not call the office and ask to speak to a pilot on the Pilots Advisory Group (PAG). They will have someone who lives / covers your location.
They know how to address this type of issue.
In the meantime I suggest you do not operate with an individual who you are sure is incapacitated. Going sick may be the only option. When the next Copilot turns up and goes sick as well then people will start to ask questions.
The CRM way out is to inform the individual that you believe he is unfit for duty.
If he intends to operate inform him you will go sick and do the following.
(1) You will ask Operations who is going to replace you.
(2) You will contact the pilot who is to replace you and inform them as to why you have become ill. It is very likely that they will go sick also.
(3) Implore the incapacitated individual to go sick.