Boyfriend finishes quick all of the sudden

My boyfriend and I hit a patch where he was able to last for about 30 minutes. We weren’t doing anything different, no new positions, anything. It seemed as he had built up stamina. Then randomly, it went away. Now he lasts about a minute. It’s frustrating both of us because he feels inadequate and I don’t really ever get anything from it. How can I help him to last longer again, and has anyone gone through the same thing?