What he was refering to is called phimosis. There are two types of phimosis. One is developmental phimosis and almost every boy has this. It is a normally tight preputial sphincter that protects the glans and allows continued development. At some time in the child's life, this will become elastic and the foreskin will stretch to allow easy retraction over the glans.
The other type of phimosis is pathological phimosis and is not normal. It is caused by infection, disease or repeated trauma such as repeated forcible retraction.
Here is an article that discusses the two types and illustrates the difference:
http://www.mja.com.au/public/issues/...w10610_fm.html
What your son has is normal and natural and nothing need to be done. Most likely he will become elastic somewhere between the ages of 3 and 7 years old. The pain he experiences with erections is normal adhesions breaking down but some areas are still attached. This is short lived and the pain is not severe, just a discomfort regardless of how he complains. As the normal adhesions break down the pain will end. Probably what he is feeling is closer to a severe itch than pain. Unfortunately, it is an itch he can't get to to scratch.
Frank