Scratch feed is meant as a treat or enrichment and NOT as a sole ration.. he will likely be suffering from nutritional deficit... specifically maybe protein and amino acids In addition to maybe some vitamins or minerals. These are the building block to life.
All though free ranging helps combat this issue.. I can see you ground is fairly barren of plant life and would suspect the same to be true about bug life. Adding a piece of cardboard or plywood, watering nicely and let sit for a few days tends to bring bugs and worms under the board right at the surface. My birds love following me around and I move those things exposing the goodies... this worked when I lived int he desert as well.
I HIGHLY recommend you get a formulated ration... either a grower or flock raiser... even a starter would be okay... anything with 16-22% protein... (scratch is usually around 8% protein, depending on which type). Any feed labeled as a SOLE ration that doesn't have over 3% calcium (preferably closer to 1%) This essentially leads to reduced immunity towards ALL things in life.
Sounds like he started a sour crop that passed.. you did well by checking that first!
If you aren't able to get a formulated ration or choose not to for whatever reason... please consider supplementing on occasion with something like Rooster Booster (brand) Poultry Cell (product).
Since wetting the feed seems to help what you are concerned about... it will also help him stay hydrated in the heat... We technically kind of keep our animals in a constant state of dehydration by our dry feeds... which we use out of convenience for storage longevity and shipping etc. What I really wanted to see was a video of the drooling when eating more than normal eating. But great job getting the video posted quickly!
If you make the switch of feeds... I can
promise you will see an overall improvement in his health, feather quality, and so much more!
Does he have any friends? Or are ya thinking about getting him some?
I have honestly never YET... in 10+ years of keeping and actively breeding chickens... seen a bird drool that wasn't either having crop/gizzard issues or on deaths door. But I do learn something (and see new things) every single day.
Hope this helps some, and he thrives fully!
ETA: sores could be a side effect of poor immunity due to undernourishment.