Welcome to HealthCareMagic.com I am Dr.Bruno Mascarenhas, and I am happy to answer your queries. I have read your question with care and understand your concerns. Let me try to help you.
Question 1 : What is the possibility of recurrence?
Answer 1 : Since he has had two attacks, the chance of Recurrence is quite high
Question 2 : Is it okay to leave him unsupervised?
Answer 2 : No.
He has two problems
It is not Okay to leave him unsupervised
Question 3 : How serious / life-threatening can this become?
Answer 3 : If he gets an attack when there is no one around, this can become life threatening
Hope you found the answer helpful.
If you need any clarification / have doubts / have additional questions / have follow up questions, then please do not hesitate in asking again. I will be happy to answer your questions.
Best Wishes for Speedy Recovery
Let me know if I can assist you further.