You can tell him whatever you want

"Would you be available the Friday before Memorial Day weekend for Jason's birthday dinner?"

"You know, I've been meaning to bring this up. He's the kind of guy whose birthday has to be celebrated. It needs to be a big fanfare, everybody pay attention and show up and kiss the ring. And as you know, I went along with it for a while. But you notice when it's someone else's birthday, he's nowhere to be found. Unless the spotlight is on him, he couldn't care less. Have you noticed that?" 

"Yah, I suppose but that's the way he is."

"Well, I'm not going to perpetuate it. He can celebrate all he wants, but that doesn't mean I'll participate. I've been very generous over the years, and the most I get out of him is a quick "HB" on Facebook if that." 

"So I take it that's a no."

"Yes, hard no."

"What should I tell him if he asks?"

"I doubt he's going to ask or even notice my absence since I'm sure there will be throngs of people ready to buy him drinks and pay for dinner. You can tell him whatever you want. I only know him through you anyway."