Tatum O'Neal's eldest son, Kevin McEnroe, is detailing how her substance abuse struggles often affected their relationship. "I call you Tatum, sometimes, because you weren't always a mom, although when you were you were at your best, and that's why you're still here, today," McEnroe, 39, wrote in a pre-Mother's Day letter shared via his website on Thursday, May 7. "I call you Tatum because your name is Tatum but also because it was a reminder that maybe, sometimes, I needed more, but that doesn't mean I'm going anywhere - I know me leaving has always made you scared.