There is a 15 year old guy who is one of my friends but feels his penis when around me?

There is a 15 year old boy who is my friend i am 13 his name is whenever he is around me i see him put his hand in his pocket and rub his penis very slily and he does it for about 5 seconds or sometimes i see him slyly and quickly squeeze it but by griping the croth of his pants he always has an erection when i get close why do you think this is