Do NOT delete the text messages. Save them to a new folder so you don't have to see them, but keep them. If he's sending you messages like this four years (at least) after you split, he is a serious nutcase and you may well need all the evidence you can get to make him desist in future.
The only way to handle this is to hit back hard. You CANNOT engage on a reasonable basis with someone who is UNreasonable. The only language someone like that understands is a brick wall that refuses to budge.
I'd send one reply similar to the advice given above, saying: "I'm really pleased that you are trying to build a relationship with our son, and I will help facilitate it as appropriate. However, my personal life is none of your business and I will not engage in any discussions about it. If you persist in harassing me or stalking me by text, I will have no option but to involve the police."
And be prepared to follow that through. Once you've sent that message, only respond to texts/calls about your DS.
Do NOT be worried about him stopping maintenance or failing to see your DS as a result. If he does that then it is clear that he has only ever seen your DS as a tool to get to you anyway, so your DS is infinitely better off without him. And you can enforce maintenance through the CSA.
Good luck. Don't let him drag you down. You and your DS deserve better. 