I don't know if this would be recommened by members at all, but take him and show him saltwater fish, but alot more care needs to be taken with them!
the other option if he is still light headed show a picture of a discus and the picture to show would be a snake pigeon type, really look for one to show him although this requires a bigger tank, but there is colour in alot show him a red tail shark no grey in that but also requires a bigger tank.
Cardinals arn't grey, maybe he is just colour blind.... if you show him colourful fish maybe he is colour blind... show him planted tank setups
Sometimes no matter what you do you can't get through the head of a sibbling... my female dog is pregnant and so happen was from my brothers dog a big mistake, I have a male and female dog, male is fixed but female isn't fixed(In case we wanted to breed her), problem was visited mom one day and my bro showed up with his dog and boom was to late before i could catch what was going on. His wife always phones are the babies here yet I want to see them etc... They want a puppy from the litter, so now its even worse then before. I have to worry about vet bills and the health of the babies and the mother, and all they could careless and only want to see the puppies. (If only my brother had of listend to me and fixed his dog I wouldn't have an issue of this matter, as his dog doesn't even have its shots! and is 2 years old) Its something more than just bothersome.(worst is this dog is around my kids from time to time I don't want to have to explain to cas why my kids need a rabie shot because of a dog bite) Your brother sounds like he has his mind set on something and wants to achieve it. It is his choice although its mean what he is doing, just remember be there when he takes a fall don't be like i told ya so.
oh and can't wait to see his face when I hand him the bills, he has no idea how much its going to cost and said he would pay half and best part was he said it in front of mom woot... means I will get my money haha, shots for all pups and vet bills for mommy dog split! HaHaHa..... maybe it will be worth him to get the dog fixed after this lol, not me my dog is 7 years old.
Sibblings will be stupid sometimes nothing you can do about it, just takes time