My son has a scab looking thin on his penis. Should I be worried? It is between the shaft and the head, right on the crease.

Not enough to ... The described scenario is not detailed enough to make a sensible advice. But, if it seems getting better slowly but surely day by day, it's okay to leave alone, and just watch and see, If not resolved as expected in 3-5 days, see Doc timely with info on its onset, progress, and changing color, size, etc, so Doc can analyze, do physicals, & do tests as needed so to deduce Dx for Rx.