You might need your vet to do a deep clean of the ears and get some antibiotics to clear it up. Along with/after that I've found these war wipes to really help with preventing it getting so bad. They're really gentle so my dog doesn't mind it - in fact he seems to enjoy having his ears cleaned now.

Pardon the Amazon link: https://www.amazon.com/Pet-MD-Cleaner-Cleanser-Eucalyptus/dp/B01IRO7WR4/