There's been several topics about the hissing waste merchant. He seems to be bugged on consoles according to some, but either way his refresh rate is stupid long. I unlocked hissing wastes ASAP and checked after ever major quest/mission completion and he never updated. I had to start farming for schematics and drops that I wanted. I posted my method that I found to update him that seems to work flawlessly but it is boring and very repetitive and still takes a long time, as his short list includes items and schematics. To be honest it is easier to just farm cradle as I can confirm all of the schematics drop from the chests there but it takes as long if not longer depending on you farm method.
But if you want to reset the merchant then What I have found that works start from the second talk too and moving forward is this
1.Port to sky hold let the game auto save and quit to desktop
2. Adjust your system clock forward 4-7 hours(been finding 7 works best for schematics 4 some times get's him stuck on premade armors) take into account am/pm and dates. If you don't then he never gets any short list items
3. log in game pick 2 merchants in skyhold to talk to(I do bonnie sims and the merchant table that has no one there)
4. port to hissing wastes and go to merchant( First time odds are nothing has happened after the first attempt of this it has worked every time for me)
5. port to 2 more locations and spend at least 3 minutes in each one of them needs to have a merchant( I do a cradle run and then go to emprise suledin keep merchant)
6. Repeat steps 1-5.
It's annoying as hell but I have all the mage/rogue/warrior armor and helmet schematics I also found 3 pretty cool uniques from the merchant table at skyhold Punched by the maker which is a good bow for Sera that applies walking bomb on hit, Pyre of the Forgotten which is a very good mage staff only negative is no "fade touched bonus", And tempest which is a griffon mace that does chain lightning. Not to mention a ton of crafting mats from resetting the sacks in cradle.
If the method doesn't work for you I don't know what to tell you outside of farm cradle, hope this helps.