Firstly after years I have finally found a great vet, Ollie is 11 so gives you an idea. It is a small private vets with two partners - she is one with a couple of young vets too. So maybe ask folk you meet with a dog can they recommend you - that was how I came to find mine.
I was at the vet this week with Ollie and spoke with her regarding flea treatment etc, was concerned about using the Sereto collar as he is a very velco dog and sits on me/next to me - she comment that while she had no feedback to her she understood my concerns, we then went on to speak about Bravecto tablet which she uses on her dogs, she said that if I googled I would encounter lots of negatives regarding them, but the only thing she has found with her and other reports from owners is loose stools sometimes for a few days. With this I declined as Ol has always had a sensitive tum.
So has Rhodri recently had a tablet for fleas, or was it topical. Have you tried giving him a bit of something else to encourage him like pieces of chicken placed in his normal food or a few pieces sausage etc, Ol has always been a monkey hungry but wanting what you have type of thing, so I will drop a spoonful of gravy on top of his food with a snip or two of meat and down it goes. If it is that he eats something else then it is just a fussy cocker
Worth trying rather than jumping to other food brands just giving a bit of enticement to what he normally has and see it it works. I had this with mine and used this method, just if it works keep reducing it each time to get him back to just eating his normal food or you will have a demanding cocker who expects the added bits to be on every time.