No, there is nothing in the rules preventing this, but here be house-rule territory.
Anything you design to accommodate the torch will be subject to your own house rules, and is highly subjective and has many options. However, consider not giving it mechanical effects, and just narrating what seems appropriate.
- The structural integrity of the shield is lowered. (Reduce AC, or make it break on a critical hit)
- The torch mount frequently catches on things. (Have weapons get stuck on it, effectively grappling both characters, or having it easily break off)
- Glare over the shield edge distracts the wielder.
Just keep in mind that a torch was designed to be a trade off for a weapon or shield, in dark areas. Another character could wield a torch, or cast a light spell on his shield. Or you could give him a magic item instead, that gives him a light effect, such as the Driftglobe.
Another alternative, making a torch in one hand and shield in the other not that bad, is to inform your player that they may use a torch as a weapon dealing the same damage as a club, or 1 fire damage.