Is it normal for a boy to get turned on from getting kicked in the balls?

me and my cousin were messing around like play fighting. he asked me if i ever kicked a boy in the balls before. i never have cuz i think thats mean so my cousin asked if i wanted to try it on him just for practice. hes only 13 years old by the way. i didnt want to hurt him but he said as long as i take my shoes off it would be ok. he says hes been kicked there lots of times before and he doesnt mind the pain. i said are u sure and he said yes so i took my shoes off and when he was ready i kicked him in the crotch but not very hard because i didnt want to hurt him. he bent over and grabbed his privates like it hurt but when he got back up he said it didnt hurt that bad and he told me to do it again and kick harder this time. i noticed he kind of had a bulge in his shorts (he was wearing gym shorts that were real loose) but i thought it was from him holding his crotch so i just ignored it. he spread his legs apart and told me to kick him again but this time he said to kick him as hard as i could. i kicked him there again a lot harder. i didnt kick as hard as i could but my aim was a lot better this time so my foot got him right in the nuts. he fell on the ground and he was like laughing and crying at the same time and he was down there for a long time, like a couple minutes at least. i kept asking if he was ok and he said he was fine. finally he got back up and i thought he learned his lesson but he asked me to kick him there again exactly the same as i did the last time. i said are u crazy and i looked down and i noticed he had a full on erection. since his shorts were so loose his penis was sticking straight out like totally pitching a tent and i could see exactly how big it was! he was so hard that there was even a wet spot were his shorts were stretched over the head of his penis. im guessing that was either pee or precum from being turned on. he wanted me to kick him again but i said no so he asked me if i would just pretend to kick him like put my foot up to his balls like i was gonna kick him but not actually really kick him but i said no way. he asked me why not and i pointed to his penis and said thats why not. he said sorry he couldnt help it. he said it happens every time he gets kicked in the balls. ive never heard of a boy getting turned on from that. is he normal?