One thing worth noting: while the studies about the possible link between grain-free and DCM are still very much ongoing, and by no means conclusive, the current thought seems to be that it's more complicated than just taurine being absent from the food. The current thinking seems to be that there may be something in many grain-free foods (possibly related to the lentils) that is blocking the absorption of taurine. The fact that Fromm supplements taurine might not actually make a difference if that suspicion is proven correct. This thread here
has an interesting discussion with several informative resources linked, and might be worth a read.
That said, there's nothing conclusive and you have to make the best call that you can with the knowledge you have. Just keep an ear out for updates, but in the mean time, weigh the pros and cons of each dog food for your specific dog. If you think you need to avoid chicken or grains, then do that! If you think it's just chicken, maybe see how your pup does on a grain-inclusive but chicken-free food. I don't know of any off the top of my head, and depending on where you live, it might be hard to find any locally, but looking into it doesn't hurt. It might also be that your pup is just fine with chicken, but had trouble with something else in the RC make up. I've known dogs who really struggled on RC or Hills but then did well when switching to something like Victor, whether grain-free or grain-inclusive.
Whatever you decide, I would take your vet's advice about the Royal Canin, Purina, and Hills with a grain of salt, though. At least, that's what I decided to do. I also recently spoke to my vet about food options for my girl, because I'd been rotating grain-free Fromm diets, and was concerned about the ongoing UC Davis study. Now, this is a new vet for me, but so far I've liked her answers to other questions I've been asking, but as soon as she told me that the only safe foods she recommended were RC, Purina (she did at least specify only Pro Plan, but still), and Hills, I knew I wouldn't be getting stellar nutritional advice from her. That said, she did help explain how complicated the current study is, especially regarding the potential role of taurine, and that was enough for me to decide that I wanted to switch my girl to a grain-inclusive, low/no-lentil food with protein traditionally rich in taurine. I settled on switching to the grain-inclusive Fromm line, but I recognize that wouldn't help your pup if he does actually need to avoid chicken.