While this is a legitimate fear that I'm sure is weighing on your mind, this is not the way to support and encourage your father on the road to recovery. In the event that your father does relapse, you should still support him and encourage him to seek the help he needs. When your father gets out of rehab, continue to be his support system.
Does your father live alone? If so, you might want to make a plan to check on him regularly because senior adults that live alone are generally at a higher risk for substance abuse. Also be sure to talk to his medical team at the rehabilitation center. Have they provided him with an individualized treatment plan for substance abuse to help him be successful? Make sure that you are also aware of your dad's triggers so you can help him cope with them when exposed to them after treatment.