Sleep Apnea Treatment

My son-in-law recently began treatment for sleep apnea. He’s 38 and has suffered from it his entire adult life, apparently. Now that he’s getting proper sleep, he has started being verbally abusive. What’s scarey is that he often can’t remember saying things he has recently said! Is there any way to get him back to normal? Is his brain having trouble making connections after all this time? We are concerned for our daughter and grandson.