I hope the doctor did not recommend circumcision, because the development of your son’s penis is normal! The age at which the foreskin will move freely back and forth varies, greatly. While 8 is older than the median age, it is still well within the average range. The tip is usually quite red. It’s because there is so much tissue packed together, when closed around the tip of the penis, and it is very densely supplied with blood. I’m not sure what you mean by the shaft being purplish. Do you mean the head of the penis, underneath the foreskin? That is called the glans, and it is perfectly normal, and desirable, for it to be purple color. In circumcised babies, it doesn’t remain purple because layers of skin cells form, as a protective measure.
The ('banjo' string is called the Frenulum. There is a condition called Frenulum Breve (short frenulum) which can cause damage during intercourse. It can make sex painful and even bloody! This requires a minor surgical procedure to rectify. However, it's normal in uncircumcised men for the frenulum to pull taut during erection and to cause the tip of the penis to bend slightly when the foreskin is pulled towards the body. Its purpose is probably to assist the foreskin to re-cover the glans to keep it protected, soft and sensitive. It's similar to the inside upper lip or underside of the tongue.