For my in-laws 75th (yes - they were born on the same day, and same year) we threw them a party inviting THEIR friends. Got a disc jockey who played big band-type music, etc. Does he have any hobbies? Like sports? If so, you could book a super box to see his favorite team play. Invite close family members to share in the event.
When my dad turned 75 - he was still playing tennis a lot - we booked a small local tennis facility and did a round-robin tournament with everyone playing mixed doubles. We brought all the finger food and drinks in ourselves and set it up in one of the observation rooms. Everyone had a great time.