major props for rescuing! you really are making a difference.
heat rash can be alot of things. but a bath with mild soap would be a good starting point. preferably fragrance free. rinse super good.
follow with cold compress (not too cold). and considering the time of year, try to keep him in the shade and AC if you can.
a very small amount of aloe vera might be helpful. he will try to lick it off and that could make her sick. so i would only use it if you have a cone (which you probably do since you rescue on the regular).
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)