If this is rare you can tell your guest your check-in time is from x to x and you could arrange an early check-in for an extra charge of $X . If he agrees you can send him money request and he should accept it before arrival. It can be classified as "extra service"
if you usually get many early-check-in inquiries you can state in your house rules that it is sometimes possible but an extra fee will be applied.
It's not an early check in methinks, it's a late check in and you should ask the guests to book the night before as well if this happens in future or offer a 'special offer' price to cover.