Your son is at the age when the normal separation begins to take place and it is not unusual to have episodes of puffiness, etc. His father probably only pulled slightly on an normal, natural adhesion that was near letting go anyway. Most of these episodes last 24 hours or less and go away. The human body has a remarkable healing ability.
The whitish matter you saw was most likely smegma and not pus. It is normal and natural and should be expected. Smegma is the greek word for "soap" and that should be a hint to you. It is simply an accumulation of dead skin cells, body oils and water. Both men and women produce smegma their entire lives.
I suspect that since today is Wednesday, that your son is already much better and the episode has already passed. Please let us know. You can expect that your son may have one or more future episodes of a reddened foreskin and maybe some swelling and even some slight pain and irritation on urination. These episodes should last 24 hours or less and should require no attention or intervention on your part or a doctor.
These episodes are a prime opportunity for a unknowledgeable doctor to strongly recommend circumcision. Don't listen to them! A second episode will prompt them to strongly insist and the fact that it has happened twice will be an indicator to you. RESIST! This is perfectly normal and natural and a part of growing up for boys. Any pain is minimal and short lived. It certainly pales in comparison to surgery and the after effects.