How do I tell my cousin he’s being too desperate?

So I’m fairly close with my cousin, he’s 23, I’m 24. He’s trying to get back with his ex, she doesn’t want him. Backstory.. they started dating when they were around 16, and dated for about 5 years, and the 4th year they had a baby. After breaking up, they both got into serious relationships, then ended those relationships around the same time (~1yr later). They’re both single now (I’m friends with her too).

They’re both good people and very good at co-parenting. She just doesn’t want him back. He posts on Facebook at least once a day about how he’s trying so hard to get his family back together and how she’s the only one he wants. After a while of this she started angry-reacting the posts, then awhile later just started ignoring them altogether and ignores all his attempts to get back together.

I feel really bad for him, and she probably won’t ever get back with him but I know he’s making it worse by constantly posting it online and constantly trying to message her. How do I tell him to back off without crushing him? Or should I just stay out of it? I feel bad because he’s definitely making the situation worse every time he posts about it. If I were her I would’ve completely blocked him on Facebook by now, my biggest turnoff is clingy-ness