There is no legal age in any part of the UK for when you may leave a child alone at home, either during the day or night, or for consecutive days.
This being so, a school or anyone else would only be justified in contacting social services if they felt the child in question was at risk, as it is an offence to leave children alone if doing so puts them at risk.
IMO if you suspect that the child in question may be at risk, does not have the phone numbers he needs to contact someone for help at need, plus knows how and when to dail the emergency services, and not to admit anyone to the house when he is alone, you are justified in voicing your concerns to the parent and offering to help.
Legally, if you know the child to be at risk, you are bound to do something to help. I doubt the best help is to involve the police or social services, unless something criminal is going on. Asking the youngster if he is coping all right or needs help would strike me as the first step, but many might see that as unwarranted interference.