If you have access to it I would suggest doing a round of psyllium on him. He may have a bit of a sand problem. While the sand can cause a serious colic it can also cause loose stools as well. Your environment looks sandy. This is not something you feed everyday but do about once a month. It is also called SandAid,Sand Clear, Metamucil(people version but can be used as well). If you treat with it you will be amazed by what comes out. Where I live I have to do this about once a month because of the sandiness here,
P.S>> the loose stools usually happen before an impaction hits. After seeing the most recent poop pics I would really push for that psyllium treatment. If in doubt put some poop in a clear container of some sort (preferably fresh poop top layer not touching the ground) then add water. Stir and let it sit for about 10-15 minutes and look for sand. My guess is you will see a fair bit of it.