What is the ruling on a husband who only performs the prayer during Ramadan?

Prayer is a great pillar of Islam, and abandoning it severs one's connection with Allah. If your husband does not pray, supplicate for his guidance, and try to advise him gently, or seek assistance from someone who can advise him. If he does not repent, then consider seeking a divorce from him, unless there is a benefit in remaining, such as the presence of children, in which case continue advising him with patience.