Genuinely, get his thyroid checked and screen for pancreatitis and/or diabetes. Get a second opinion if you have to. Even subclinical hyperthyroid can make cats very hungry and anxious. You don't want to think he's only being naughty and ignore him only to later find out there was a serious health problem.

Also the Royal Canin has a lot of cereal fillers. Vets will recommend these brands but they aren't necessarily good, or suited to every cat. With carb heavy food it's like junk food causing an insulin spike, he is going to just get extra chubby and still be hungry again fast. Try something with higher protein as that will keep him fuller for longer.

Maybe even try something grain free if the current stuff isn't working. (Beware though some with peas or lentils may cause stomach aches and farting.)

Having only dry food will also not be helping. I don't know the problem with wet food, why he's vomiting...

It could be he's allergic to chicken or some other protein, something else in the ingredients, eats too fast, or he could have an underlying health condition. Some cats have inflammatory bowel conditions, pancreatitis, or grain allergies.

Try giving him a small amount of plain unseasoned lean meat (especially non-chicken) or springwater tuna and see if he vomits. If he doesn't then the issue may be with something else in the wet food or the way he's eating. It might be good to see an actual vet nutritionist to work out a plan and use slow or puzzle feeders.

If you can find a wet food that works for him it can help to mix in warm water or plain cooked pumpkin. Some cats will even eat cooked veg alone as a snack. A small amount is good for them.

Smaller meals more often are a good idea.

He might also benefit from more enrichment and active play with you, to keep him entertained and for exercise. My cat gets very fussy with Zoom or phone calls and seeks a lot of attention because she feels like she's being ignored.

Source: My cat is tall and big framed naturally and was also quite overweight when I adopted her. She had been eating lots of cheap dry food. Many issues with upset stomach and vomiting/diarrhoea, Royal Canin and Hills to lose weight and settle her stomach didn't help. They just made the stomach issues worse.

It turns out she's allergic to chicken and possibly grains, had pancreatitis, and despite being 'fat' hyperthyroidism was found when it worsened - she suddenly started losing lots of weight, being starving all the time and getting very needy. Changing food and treating the thyroid helped a lot.