I've never fed game bird, so don't know what is in it. I do know that all flock, which is cheaper than the starter grower, is about the same protein level as the starter/grower, and since you already have the oyster shells, it should save you some $$$...my feed store is about 5 bucks cheaper, for the all flock, than the starter/grower..a washed out tuna can nailed near the feed, will work fine for the oyster shells for your layers. You might want to give your vet a call and ask if 3 weeks of layer, could cause kidney damage? and if so, how much and what you should do. They should be able to tell you over the phone, if not the vet, your local extension usually have some great people that will talk your ear off, about poultry and could give you the info.
Is there any way he could have cocci? I know, from what you explained, he is not showing the classic signs, but they all don't, that is just a guide line to go by, mine didn't show the typical signs until they were really really sick...just a thought, as cocci seems to be running rampant this year! Even in very dry places and with very non typical symptoms to boot. I can't hurt to try treating for it, as it is only a thiamine blocker, not a "true" med, but I would find out about the kidney issue, first...before treating for anything...just and FYI, if it is cocci, once they reach that tipping point, they go down fast...like in dead. I bet you can get some good info, over the phone from your vet, or the extension...I would try that first! Best of luck!
Oh, one lady recently, had a necropsy done on one of her hens, that died, that had a pale comb, all her chickens had pale combs for that matter and it came back cocci, so far, the rest they are waiting on, to see if there are other issues...another FYI.