From what you say this boy has never been taught any manners, or else his parents gave up the attempt when they found it unproductive.
I assume he is the elder of their two children and they realised their mistake in time with his little sister.
Honestly, there is no way of changing a 15 year old - most of us were slightly rude at that age, and as this boy has always been like this, it is far too late to do anything about it, within the family.
Also you quite rightly say that you do not wish to alienate his parents, which will be the result, as like as not, if you mention his lack of manners to them.
They too seem lacking in empathy if they expect expensive presents or allow their children to do so, when they know or should do, that your income is less than theirs.
I suggest you mention either to your son, daughter-in-law and both children when the family is gathered together, or failing that to the two adults, or whichever you feel most comfortable saying it to "I'm sorry to say, that due to the rising cost of utilities, food etc. I shall have to stop giving the usual kind of presents, so there will only be small token gifts from me until things get easier again."
Alternatively, you could simply say that due to the present inflation and price of basic commodities you are unable to give any presents from now on, and naturally don't expect any either.
Don't mention his lack of manners at all in this connection, and don't let yourself be drawn into a discussion of present-giving. Deliver it as a statement of financial fact, then change the subject.
If the family attempt to discuss this, say that it was not meant as a discussion, you have review your present income and outgoings and this is what sadly is necessary.
To be fair you will have to say the same to your other grandchildren and their parents.
I would not give t oken gifts at all - my mother did so in her latter years, and I honestly would have preferred her not to, as we received completely useless things that had cost her money, even although very moderate amounts.