Hi okay several quick questions

1) My horse is bit shy, what are some training tips to get him to lower his head when putting in the bit?
2) Is it true if your horse is HOT not WET, but sweaty and hot from a long ride you walk him out before putting him in a paddock with water. OR is it okay to put a cooler on the horse and put him in a stall with water still. Or should you always walk the horse out and why

3) Today i noticed on my horse on the sides of his mouth were the bit is a tiny bit of hair and fur was rubbed off. First will he scar, second we dont know why. I was not pulling hard or anything. He has been in a hackamore for most of his life, he is 7. So does he just not have calasses built up?
THANKS