Here are some questions that need to be answered first:
Are you sure he is gay? nothing in your description mentions that.
Are you sure that he is over the age of consent? realy you need to be extra careful about that.
Suppose that he is gay and legal. Had he any sexual experiences before? The answer to this question leads to a different approach.
Now, approaching him depends on many factors, for example how open he is to older guys, the environment and social cycle etc. If you the town you live is a small town, and not very gay friendly, this could have severe consequences. 18 is still considered too young by some parents and it may cause drama.
Check his social media status and try to figure out were he stands. This is a good and safe start.
Again, be sure that he is over the age of consent.
