Hello @Trent-and-Kijana0
If he hasn't booked to stay with you after the holidays I wouldn't be inclined to keep his belongings in your guest room or leave his car there.
You can always tell him you have family visiting and that's why you blocked out the dates so you will need the guest room.
Are there left luggage facilities in your area that you could direct him to?
You know the guest and we don't, so only you can make a judgement call on this.
Perhaps a compromise if you have room is to say that he can leave his car at yours for XXX days, but will need to use a left luggage facility for his belongings. (You can tell him they won't be covered by your home insurance as he won't be a paying guest during this period).