I think it depends if you want him as a co-host or a guest.
If co-host, you could say “I’m planning D4’s party. She would love it if you were there to celebrate with her. If there’s anything special you’d like to contribute I’d be happy to incorporate your ideas.”
If guest, you could just say “D4’s birthday party is on x date at x time. She would love it if you were there to celebrate with her. The RSVP date is x.”
I’d avoid anything that asks him to do something ie. please let me know xyz. Just let him know what you doing and that he’s welcome to contribute. That way, what you do isn’t dependent on his decisions.
