he fancys the butt off you
yes, this is very normal for males of this age to respond this way. It is because of male hormones. At that age a slight breeze can give arousal.
yes, it's totally normal. Especially if he's attracted to you :)
a guy at 21 can get turned on if the wind is blowing in the right dirction
he finds you attractive, take it as a compliment...and if you like him then just "take it"
Yes, especially at that age.