AIBU to expect a more polite response?

Firstly, apologies for the length of this, but here goes: I messaged my 2 DGS this morning, to ask both if they would like to come out for a meal with their Dad (my DS) and me as it’s my DS’s birthday today. To give a bit of background: their Dad and Mum separated about 18 months ago, the relationship had been becoming more and more toxic several years before, then came lockdown, which made the situation far worse. His ex partner is a very difficult woman who has mental health problems, which seemed to grow after each pregnancy. My DS, (+ all our family and her family) has done everything he can to make her life as easy as possible because she was the main breadwinner in the house, but nothing he ever did was enough. Unfortunately, she encouraged both the boys to criticise their Dad, and would constantly put him down in front of them. The end result of this is that the elder of the two (13) doesn’t want anything to do with his Dad. The younger (11) decided about 6 months ago that he did want to see his Dad, and so manages to see my DS every other week, unless she has decided to arrange something else for him. His ex partner won’t talk to him at all, insists that all communication is done via email, and then berates him if she hasn’t seen the email in time. I’m only permitted to contact my DGS via WhatsApp, and that has only been in the past 6 months. There is so much more to the story, but the bottom line is that she is in control of everything that the boys do and probably say at the moment, and I totally understand how damaging and toxic their situation is at the moment and have complied with all her demands so that I don’t make it worse. The reply to my invitation for the meal from my eldest grandson was a stark ‘no’ - that’s it!! I had a reply from the youngest asking if his Dad’s birthday was today, I have written back and said yes and am currently waiting to see if he is allowed to come out for a meal tonight. I am debating how to respond to my eldest DGS - I looked after him from 6 months old and played a big part in his life up until lockdown. In normal circumstances I would not let him get away with just replying ‘no’, which I think is fairly impolite especially to his Nanny - and what I would like to say is ‘Well that was a little impolite darling, perhaps a no thank you would have been better, but thank you for replying ‘ so that he knows a one word answer is not polite; but……. I feel so sorry for him having to cope with all of this at such a fragile age of hormones raging etc. So, do I send him a text just saying thank you for the response, or do you think it would be unkind to gently take him to task for being so abrupt? I know it’s daft to even worry about it but I’m torn and so sad - he was such a lovely polite and fun little boy and we would chat for hours, and now he’s a morose, rude bad tempered young man that doesn’t reply to any texts (from any of our family, Uncle or cousins included). I always reminded my boys as they were growing up that manners cost nothing whether spoken or written even when they went through the ‘teenage’ phase. Thank you if you got this far , and I hope it makes some sense. Any suggestions as to what I should reply?