I have an question: Is it normal for an 13 years old boy to have his penis "head" inside the foreskin when he is erect? When my penis is erect it doesn't look like other men's penises. Is it normal?
That there are guys who find themselves with too tight of a foreskin isn't rare, but it isn't considered normal or what should be happening. The condition is called phimosis. Full phimosis is when you can't expose the glans (the head of your penis) even when you are flaccid. Partial phimosis is when you only can't expose the glans when you are erect. It is estimated that between 1 and 5 percent of 16 year old uncircumcised boys have problems with phimosis [Phimosis and Paraphimosis].
There are different treatments for phimosis. See: "I'm uncircumcised. The skin on my penis doesn't pull back all the way to release the tip of the penis. I think it should, but I don't know if I should see a doctor"