I would call the vet to make him aware of what happened just to be on the safe side. This way if you start to see any odd behavior in your dog you can take him in and it won't be a total surprise to the vet.
I did find that when this drug was being tested, a study was done in dogs where they were administered 105 mg/kg per day orally. Your dog only ate 5 mg. That's a pretty low dose. The drug CAN lower the blood pressure and heart rate but again, watch for odd behavior. You will know if it is affecting your dog in an adverse way. It MAY cause him to urinate more frequently than normal, that's one sign you may see.
You can also try calling a poison control center. 1-800-222-1222
Please let us know how he is.
Ghilly
As you probably know, Bisoprolol is a beta blocker...it can slow your heartbeat, and lower your blood pressure. It may or may not cause the same reaction in your dog.
Your dog may be showing signs of lethargy..( tired ).. and weakness...and may need to be monitored by a vet.
It would be best, if you brought your dog to the vet to be evaluated, or at the very least, call the vet, and find out what they suggest.
Talking to a vet, will put your mind at ease, and the vet will let you know what type of intervention or supportive care your dog might need.
I hope things go well for you both...
Connie