@Tal5968 Your listing does not specify a cutoff time for the check-in window, so your guest is well within his rights to assume he can check in anytime after 8:00 AM. I strongly suggest revising your listing right away to reflect your actual availability for check-in. If you're accepting same-day change-overs, the check-in time will have to be later than the check-out time for obvious reasons.
If you have time to organize it now, you can organize a lockbox that the guest can access for self-check-in. Or for a more welcoming experience, you can ask a friendly night owl in the area to be your co-host for the day and greet the guest in your place. Either of these options would leave your first guest with a positive impression.
The bad alternatives would be to cancel the booking with penalties, or to ask the guest to agree to a date change for a next-day check-in and refund the first night, forcing him to find somewhere else to stay for at least the first night. That wouldn't put your hosting life off to a good start.