Self Assessment for a minor

My 14 year old son has been selling digital items on my eBay account for about a year and any payments received have been transferred directly to his bank account. Since February sales have rocketed and we were contacted last week from eBay to say I needed to upgrade to a business account. Having looked at my son's bank statements he isn't making as much profit as you would expect due to his outlay, however, as it is not possible for him to have his own eBay account (due to his age) I became concerned about the implications this could have on my tax allowance so made a call to HMRC. After the hour long wait to have my call answered I was informed that he would need to make a self assessment and all the information was available on the gov.uk website.....great. What they didn't tell me was that to make a self assessment he would need a national insurance number and at 14 years old he doesn't have one. Can anyone tell me where we go from here please? I feel like we're going round in circles but don't want to discourage him from trying to make some money,TIA.

Hi Rachel Plant,
HMRC would have to manually set up a Self Assessment record in this case.
You will need to complete SA1, fill in, print and post the form to:
H.M. Revenue and Customs
Self Assessment
BX9 1AS.