Hello shamira;
welcome to HealthcareMagic
Allergy to
paracetamol is not seen commonly and if your son has it and due to that he has been left with a dark spot on neck.This may resolve on its own in course of time may be few months and there is no proper medicine to lighten it.Just in case you want to try you can apply Eldercoid ointment on that specific part as it is a
steroid and may help to lighten it;another option if you want to apply herbal medicine you can apply
calendula ointment as it will be safe(it is a homoeopathic ointment and is available at chemist shop.
Thanks