Both of those dry cat foods are very high in carbs. Royal Canin DD dry is 38% and Hill's W/d is around 37%.
My foster cat Wink was on the Hill's W/d dry. It took many of these
transitioning dry food addicts from dry to wet tips to get him to make the switch. I probably used 90% of those tips and tricks in that transitioning article. It took me about a month of a slow changeover and decreasing the amount of dry available while increasing the wet available. And lots of patience and persistence. But it was worth it, and Wink is still OTJ 9 months later.
At first, Wink did not even recognize the wet food as something edible. ohmygod_smile He turned his nose up at
every flavor I tried until I gave him the Fancy Feast Turkey & Giblet flavor. He loved that. Would not eat any other flavors for about a month. I was so afraid he would suddenly decide he hated the T&G and then where would we be. So, I slowly introduced other flavors, mixing in a tiny bit of a new flavor with his favorite T&G. He now eats about 6-7 flavors but still hates all the fishy flavors.

:shock:

Go figure. Maybe you have not hit on the right flavor to entice him yet.
For now, your cat needs to eat most importantly. So be sure he will eat something, even if it is the higher carb dry food, and the switch over to a better low carb food may take some time. I thought the month that it took Wink to switch over to the wet food was really fast.