How to get rid of Houseflies if my dogs ears? ?

This is just the first in the sites that came up when I googled your question.

(cp) Spray your pet with Mycodex All-in-One Spray to repel flies. It is water-based (most sprays are alcohol-based), so it can be used on the head without irritating the ears or eyes. You will need to apply it daily until the ear is healed, then as needed to keep the flies off.

Fly Bites on Dog's Ears - Revival Animal Healthwww.revivalanimal.com › flies-biting-my-dogs-ears › learning-center

Keep cleaning the cut (I'd use a mild Hibiscrub solition) and KEEP THE DOG INDOORs other than for toilet needs.

He may benefit from a course of Antibiotics to aid healing.

FIRST you have the vet check the ear to make sure it  is healing correctly and doesn't need treatment , there are several things you can use ALTHOUGH not on broken skin like a cut and with an older dog it needs a vet check first as if it is from a rose bush it could cause Tetanus which is a potentially life-threatening disease in dogs seen following infection of a wound or surgical site by the spore-forming bacterium Clostridium tetani. Subsequent production of a potent neurotoxin results in local or generalised disinhibition of motor neurons and sustained muscle spasms.

Keep the dog 8ndoors as much as possible.  Have a vet check the ear for infection.  When treating the ear after cleaning it they usually put a strong antibiotic cream over the area to keep the infection out, it also keeps the flys from wanting to bite.

Stop leaving the dog outside.

I never see flybitten ears on indoor dogs.

The wound may also be infected if it is attracting flies

I'll bet a Veterinarian would know.  Call one.