Self assessment fine for son who left uk last year

Hi My son left the UK April last year, he informed his accountant at the time and was under the impression that as he hadn't traded as self employed since the previous tax year of which he did his self assessment his accountant would tie up any loose ends, now I have received a letter from HMRC addressed to him saying he has been fined for not sending his tax return on time, does anyone know who I would contact to inform about this, he is currently in Indonesia so can't phone himself due to cost and can't email, I did email his accountant but haven't had a reply ( guessing he isn't interested with my son no longer being a client ) even though my son won't be returning to the UK he doesn't want HMRC chasing him when he hasn't been self employed since January 2023 and his business ran at a loss for the year 2022/2023 any help as to which department I need to ring would help tremendously

Hi

If your son ceased Self Employment in January 2023, he will have been due to file a tax return for the 22/23 tax year.
This will have been due by 31/01/2024 - if you have just received notification of penalties for an overdue return, it will probably relate to the 22/23 tax year.
If your son cannot contact us by phone or webchat to discuss this, he can write to us at HM Revenue & Customs, Benton Park View, Newcastle Upon Tyne, NE98 1ZZ.
You can contact us by wechat or phone for general advice at Self Assessment: general enquiries - if you would like to discuss specifics regarding your son's record, he can confirm this authority by writing to us at the address given.

Thank you