Hi Sir or Madam,
The dosage for benadryl is up to 2mg per pound every 8 hours. So for a male lab at 10 months old, I don't think you have even given 1mg per pound. So if this is due to an allergic reaction, you need to give at least 1mg per pound. I'm going to go over the various causes of facial swelling.
Facial swelling can be caused by infection, an allergic reaction to a bug bite or sting, bruising, mouth problems, cancer or muscle inflammation or due to acetaminophen toxicity. If it is possible your dog was bit by a snake, you will need to take the dog into the Vet immediately.
If it is due to infection you should find a cut or puncture somewhere. This swelling will often be warm and painful and your dog should have a fever normally with this type of swelling. Your vet will need to take care of the injury and possibly start your dog on antibiotics.
Swelling due to an allergic reaction is frequently caused by an insect bite or sting. Often the whole head will swell and it is generally not painful though your animal may be itchy. I've givien you the dosage for benadryl and if it doesn't your vet should be consulted.
If the swelling is associated with acetaminophen poisoning (Tylenol) you often see itchy paws and your dogs gums may appear brownish. See your vet immediately.
If it is caused by a bruise then a trauma would have occurred and in light colored animals you may be able to see a discoloration of the skin.
Certain cancers can develop on the head causing swelling in a specific area. Swelling under the jaw may be a swollen lymph node.
Inflammation of a muscle is another possible cause for a dog's face to swell and typically it is the lower jaw or the top of the head that swells.
An abscessed tooth can sometimes cause lumps or swelling of the muzzle or jaw. You will want to check your dog's mouth and look for any broken or discolored teeth. If you find any, buffered aspirin can be given to a dog with a dosage of up to 5 mg per pound every 12 hours for pain until your dog can be seen by your Vet. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet.
As you can see there are many causes for swelling in the head and most will require that you seek medical treatment.
So I would give more benadryl to make up the rest of the dosage, but if that doesn't help, you need to have your dog seen as swelling of the eyes can cause problems with your dog's sight if not brought under control.
Hope this helps.