Hi, OP -
While I agree you cannot ask about your DS’ grades or anything else, you can definitely provide information.
Occasionally the parents of my personal tutees have contacted me to express concerns. I tell them up front that I will not be able to share information but I don’t mind listening. If their concerns are consistent with mine - for example if the student’s work has recently deteriorated or their attendance has badly fallen off, etc - I may ask the student to call in. If the student doesn’t and especially if attendance and turning in coursework continue as red flags, I have asked our School Student Support Officer to do a Welfare Check.
If you know the name of your DS’ personal tutor, you could email them with his name in the header. Make it clear you are not asking for information, but briefly express your worries, with evidence. I would not use the lack of discussion about marks or grades. You have seen on this thread how that can be (mis)taken. The other stuff is more important. If the evidence sounds feeble, reflect that your worry may be exaggerated. Cancel the messsge!
If the message sounds reasonable, ask the PT to reflect on whether they see concerns and, if they do, could the PT please take this forward?
If you don’t know who the PT is, you can write directly to the Student Support Officer or similar in the School, asking for a Welfare Check. That is a bit more drastic however.