Should a 50 year old is having anxiety attacks ring alarm bells? What is he not telling me?

Many causes. Your question seems to imply you think he is having panic attacks now because he is doing something wrong like cheating on you. Although possible, there are many reasons why a man may first develop panic attacks at age 50 without doing anything wrong or having something to hide. Many men become more anxious as their bodies become older and less able. Growing older can mean facing our mortalitly.
He may not know why. Although women are more likely to have panic attacks, men do have them, and they can have them at any age. The effects of anxiety are both cumulative and situational. He's experiencing some difficulties, but he may not even be aware what the sources of his anxiety are.