It's not angelfish in particular but instead it's that your tank is infested with ich. The fish that currently reside in the tank likely have developed a resistance to it while the angel has not.
What should I do to keep it alive?
First I would definetly quarintine him and if possible treat the whole tank for ich
I also did a freshwater dip for 2 minutes and that clear just about all the spots off its body!!
I also did a freshwater dip for 2 minutes and that clear just about all the spots off its body!!
But this is only one of the life stages of Ich. There is most likely more living in the sandbed of your tank and will get back on your fish once it returns to the tank.
How are you "treating" your display? Nothing other than leaving it fallow is needed.