Hello everyone and thank you for all your answers.
He is 21, and the day he got this 'warning' the police made him spend like 10 hours on jail. They didnt ask for any fine or something.
He do hold a spanish passport, as well as me but I really dont understand about this things. He was saying in the future he wants to get the british passport as he says he can benefit more but I dont really know, I'll have to ask him.