As kno4te suggested pics would help as there are two diseases as far as I know that can appear to the untrained eye as white spots. The most common is ich which is probably what it is, less common is Velvet disease but Velvet is often more gold in color and is more fine appearing almost like dust. While ich looks like as some describe looking like salt grains.
As far as I know ich is only vulnerable in it's Theront stage of it's life cycle. People raise the temp to increase ich's life cycle to get to it's Theront phase. It is then that salt will kill it. Ich can be cured with just salt but it usually will take longer than if you used meds in conjunction. If you do choose to use meds you'll have to remove if your using it any carbon in your filtration.
If you change the water just realize you will also be removing some of the salt you put in. Choosing to change your water will be dependent at least for me by when you did your last change and what your current water parameters are.
Flowerhorns are not generally shy, there maybe something else going on. But if it is stress providing him with some cover would help a lot as he'll feel more secure knowing he has a place to retreat to.
Edit: Also be sure to keep the water well aerated when raising temps and using meds. 28 Cel should be sufficient, you don't want to raise it too much as warm water has less dissolved oxygen in it and some ich strains are temperature resistant.