First of all, you don't say how old he is. The older we get, the harder it becomes to recover from exercise injuries. You also gotta understand dead lifters -- people who strive after things that don't matter, such as big muscles, or to lift more than someone else, well, you really can't reason with them. Or I should say us. I've exercised myself into all kinds of pain because it's my outlet from the anxiety. I'm not sure he needs an MRI, actually, but only getting one would tell him that for sure. However, when you come back from an injury, you have to start back slowly, you can't just go back to where you were. I'm also guessing he's doing a weight that's a problem for him or without sufficient spotters and precautions or he wouldn't have had the accident in the first place -- he got overconfident. Lower back injuries can take a long time to get better, and regular docs aren't the best people to get exercises from -- physical therapists or physiatrists or orthopedists are the specialists in this stuff and they will demand pictures. But the fact he's feeling this much better seems to indicate it's not a serious problem -- you know when the lower back is out, believe me, I've been there and it took a long time to come back. But he's got to build back up to where he was, and he's got to properly warm up and take more precautions than before this happened. But you know, we can't make him not be stubborn, it's also the thing that allowed him to get to where he could deadlift that much weight.
Stop the lifting period . he will get over it and his body will recover. We all get old . do a different sport , exercise good for the heart and mind . he wont regret it .hope that helps.