Is it normal that my son's friend get hard on when he watch me or he disrespect me?

My Son's friend comes thrice in a week at our home to study with my son. He is coming from 1 year.

He is a nice person and respect me a lot. He help me a lot. Not only me, but to other members of my family too. We trust him a lot.

But I always notice that he gets hard on when something unexpected happen with me.

Like, when I bend down to raise something and my inner body gets visible to him (not always, but sometimes when I forget to cover my inside), then he don't move his face to something else, he keeps staring there and gets hard on inside his pant. Same thing happen when I touch him by mistake. That's why, I try to stay away from him, but as because he is very respectful, I reply very nicely whenever he ask me anything.

I don't know what he thinks about me, but he never tried to flirt with me. He never crossed his limits, but sometimes he gets hard on while talking with me, whether anything happens or not. Anyone can catch him, but we can't say anything because he has not done anything wrong yet. I don't reveal or expose my body parts. I cover my body properly.

I noticed it many many times. I can see that easily, because it's clearly visible when he gets hard on. It's more than too much.

Is it the sign of disrespect or it's normal? Is he trying to disrespect me?

Please help.