Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This medication could cause a significant drop in his blood pressure which could be very serious for him. You have a 2 hr wndow that you can safely remove this by inducing vomiting using 1 teaspoon of hydrogen peroxide per 5 pounds of your guys weight. Wait 15 minutes and if he has not vomited, do it again...you can safely do this 3 times.
Try very hard to find your guy....perhaps call your daughter to find the boyfriend. If the 2 hr window is past, have teh boyfriend take your guy to your vet for an exam.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta