Have you tried feeding the one stealing the food out where he is comfortable, and put the other cat up to eat? I have to separate my two boys, because the little one will inhale his food in 30 seconds and eat the others.
If you are used to free feeding dry food, I would seriously consider changing that. Like others said, it will lead to obesity and other issues. Feeding Your Cat: Know the Basics of Feline Nutrition :: healthy cat diet, making cat food, litter box, cat food, cat nutrition, cat urinary tract health
is a great website to read about why cats do not need to have access to dry food 24/7 and the problems that it can cause. You want to establish mealtimes, it is not good for a cat to have access to his food bowl 24/7. I know that seems to have worked for your cat, and he is a grazer who likes to nibble on his bowl throughout the day, but it would be in his benefit to change that habit with regular mealtimes.
With your older cat being annoyed with the new younger cat, we had this problem too, even though Taffy (older cat) and Stephano (kitten) got along OK, it was clear that Stephano was annoying the heck out of his new brother, so I was sure to put him up at times, and give Taffy a break. My boys have their own room that they sleep in at night, and are in while my oldest cat is out....she cannot be out with them since she is vicious to other cats. So my boys have their bedroom, with a cat tree and window and litter box and are very comfortable, and they know the routine. That is the thing, at first, if you give the kitten a time out from the older cat, he is going to be mad, mew, scratch the door at first, but eventually they settle down and if you make it a regular part of the day, they get used to it. My cats know that they get their PM meal at 9:30 (right before bedtime makes kitten sleep MUCH better) and then they get put up for the evening in their room. Stephano mews for a good 5-10 minutes in protest, and then he curls into his tree and goes to sleep. It is routine now, and he is used to it. He now doesn't have to be separated from Taffy, but while he was young I did frequently put him in his room and give poor Taffy a time away from the little pest, that way he could sleep without one eye open and have a break. I think this would work wonders for your cat, to have a break from the new kitten. I know new kitty is going to protest and freak at first, but if you have a litter box in there, a comfy bed, maybe you go in there with him, pet him, play with him, get a laser pointer.....best thing ever to wear out a little kitten and get him tired, he will eventually settle down. I think the best thing I ever did for my older cats sanity, was give him breaks from Stephano while he was a kitten. And then when I did let them out together, they seemed to be happy to see each other and play more.
Some feliway might be a good idea for you, I know people like the plug ins but I have the spray, I just spray a spray of it in the rooms they are in every day, and I think it really helps.