From statistics, around 25-30% of men have problems with foreskin or frenulum development. Often these problems are insignificant and don’t create any problems on everyday basis.
In situation described in a letter – during intercourse or right after it there occur pain in frenulum area. This pain is caused by frenulum being too short, because of what, it gets traumatized during intercourse.
In normal situation frenulum should be long enough so foreskin can pull off completely from the head of the penis in erected state and even out on penis. At a puberty age with development of penis and foreskin, frenulum should develop long enough also.
Very often short frenulum combines with partial phimosis of foreskin – narrowed ring of the foreskin. At partial phimosis, you can pull off the skin off the head of the penis only when it is at rest state and you can do it only partially when it’s erected. In such a situation it’s hard to tell what is primary reason – is it this shortened frenulum which doesn’t allow foreskin to slide off the head of the penis and therefore there is not developed enough ring, or is it this undeveloped ring which doesn’t allow frenulum to evolve.
In most cases there is seen situation mentioned in above letter. Frenulum seems to be in right size, but at bigger “load” it troubles movement of the foreskin as a result it is stretched more and traumatized. This basically is the reason why your boyfriend is having pains sometimes after sex.