Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for Angus. It certainly sounds as if he's having an allergic reaction to something...possibly an insect sting or spider bite (the most common causes for swelling of the face).
Benadryl is exactly the drug which I'd have suggested in this situation but I believe your dose may be too low to be effective. I recommend 1 mg per pound of body weight which is going to be about 75 mg for a 60-70 lb dog.
If the label says there's 12.5 mg in 1 teaspoon of liquid and you gave him 1 teaspoon, then he's going to need more drug for this to be effective. In fact, I typically don't recommend liquid Benadryl for my larger canine patients since you'd have to give a 60-70 lb dog at least 6 teaspoons or 2 tablespoons which is a lot to get into a dog.
I prefer the 50 mg oral tablets and 1.5 would be given to a 60-70 lb dog.
Sedation at appropriate dosages can be seen.
Most patients will respond to antihistamines although there are a few who will require steroids before improvement is seen. If the swelling hasn't resolved (after the appropriate dose of Benadryl has been given) after a few hours, then I'd give him another dose in 12 hours.
I hope this helps. Deb