Not sure what state you are in, but a similar book to this one called “What Every 18 Year Old Needs to Know about California Law” (only for your particular state) would be very helpful, especially if he is a partier. You can get in a lot more trouble for stuff when you’re 18 than when you are considered a minor.
For fun, you and your friends could take him out for dinner at Chucky Cheese and then give him a bunch of silly gag gifts.
Since wine gets better with age, but he is NOT OLD ENOUGH TO DRINK YET, you could talk with your local wine merchant and have someone who is 21 buy an affordable bottle of wine that was vinted in the year of his birth and put it into some type of box (purchased or made) that states that he is to keep it un-opened until the date of his 21st birthday.
In most states in the US that have lotteries, you only have to be 18 to play, although in a few states you still have to be 21. Have someone 18 or older buy your friend 18 lottery tickets (but decide ahead of time whether you will let him keep any winnings or if he has to share them with you or the person who purchased the tickets)
Find out from his parents, other friends and siblings what his favorite 18 foods are and have a party and serve those foods. He’ll be thrilled! Number them so it will be obvious that that is what you are doing.
Buy him a nice copy of the US Constitution (since all of his rights and responsibilities will be spelled out, and MOST US citizens have a very minimal understanding of exactly what is in the Constitution, even though every law that we have to follow (or attempt to change) is held to the standards of this document). Then take your friend to register to vote, if he hasn’t already done so. Politics and voting and knowing and understanding the laws of the US is boring, but understanding all of this and participating fully is the only way to ensure that “Government by and for the people” happens. Then bake him a cake that looks like the US flag. Yummy.