I am so sorry to hear about Micky
I suspect that Meeko liked being the only cat after losing Micky and now he has this horrible little burgglar running around his space and he hates it.
I suspect the age difference is too big but it just maybe that Meeko doesnt want another cat around, very hard to tell.
Have you tried a feliway plug in as that may help.
I have two birmans who are hated by my other two cats who were here first but the birmans tend to live upstairs and Misa and Sasa downstairs. In last couple of weeks Misa has decided he wants to sleep on the bed and he and Franta try the best to get each other off the bed...........no fisty cuffs cos we are long past that stage but Misa who is a big boy will deliberately try to lie on one of the others.
I so wish, despite being in love with the birmans, wished i hadnt bought the second pair in cos we had months of nastyness but so peqaased it was a pair who knew each other all their lives............litter brothers.
Sometimes cats will learn to tolerate each other although they hate each other like mine,,,,,,,,,,,,if feliway doesnt work, the only other thing I can suggest is referral by your vet to a behaviourist like Vicky Hall.