Hi, If the music if protected by copyright it is illegal to download it. I suspect your son is not after music that is free of copyright restrictions. We give rather less straight answers to this question sometimes, but as it is your ip address and computer you should know the facts. In the UK I believe prossecutions have been made, mainly on people who are sharing or uploading a LOT of music. I don't think the person who downloads a small amount of music is likely to be prosecuted
I hope this helps