If someone is baptized Catholic he is considered Catholic. Essentially the view of canon law is: once Catholic, always Catholic.
In the scenario you described you did not state whether the individual had received first communion or confirmation. If not, then the individual should contact his parish to enroll in the RCIA (Rite of Christian Initiation for Adults) process to receive those sacraments. Since he was baptized Catholic he’s not considered a convert, but rather just a Catholic adult who hasn’t fulfilled all the sacraments of initiation.
If he has received all three sacraments of initiation in the Catholic Church (baptism, 1st communion, confirmation) then all he needs to do is make a good confession and he can receive communion again.