Hi XXXXXXXXXX~
The dose of Benadryl is generally 1-2mg per pound...so technically your dog could have up to 50mg of Benadryl every 12 hours.
If we go by that general dosing instruction...then no, your dog did not get an overdose and it shouldn't be dangerous in the least.
He'll likely be really sleepy (more than usual), but just keep your eyes open for things like vomiting or difficulty breathing...if you see any of that, have him seen by an emergency vet as soon as possible...but I doubt you'll see anything to worry about.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.