Hi, I guess you keep your expectations low, or work out whether it's worth accepting invitations from him. Is he always late, or is it a once or twice off? It would be easy to call each other when you are running late, in fact it's just good manners. But you did say he was always letting you down, so it seems like a pattern.
I think you did the right thing in asking someone else to the function; you don't have to rub it in, just state it as fact.
Make an effort to act politely to him and maybe it will rub off. You can hold your own without 'nagging' (I hate that word).