So a salt block is actually designed for cattle and livestock and not horses. Their tongues can’t get enough salt from that. I would suggest adding loose salt to the diet to ensure enough drink.
Grass or hay should be sufficient as long as its high quality and available 24/7. Some horses do have issues digesting grass though and if there are allergies that might be something to try, an all hay diet instead.
I would also suggest something that might not be considered and that is to use the Mastersons Method on this horse. I personally have found this method to be beneficial in many ways but one thing it does is get the moving and making great gut sounds. It’s easy to learn and you can do it yourself you just have to be observant and patient. It likely won’t cure the horse but it will help keep the gut moving along nicely. When I do my horses they all have a great big drink during the session which also helps.