If his shoulders are at the top slots, he still fits. However, tomorrow his shoulders could be over the slots, or it could be next week or next month. It's hard to tell when most children will have their next growth spurt. For my own child, I'd prefer to buy a new carseat now. A Forward Facing carseat can be used if 1-Child is withing the weight limits 2- The harness slots are at or above the child's shoulders 3-The top of child's ears are not above the top of the carseat. You must meet all 3 of those to continue using a carseat.
These pictures are of children properly using a carseat forward facing. They are within the weight limits, the harness slots are above their shoulders, and the tops of their ears are well below the top of the seat.