As the teacher you have many opportunities to teach your students about what it takes to BE a good friend (to oneself and to others). The obvious ways are sharing stories with your students that lend themselves to discussions about friendship... choices the characters make that reflect "real" friend behavior vs. those choices that reflect behavior that is a)respectful b)unkind c) rude d) all of the above. You can also create projects that require small groups working together... cooperation is often one of the seeds that can blossom into friendship.
Book 2 of my award-winning Middle School Confidential series is called Real Friends vs. The Other Kind. It's also about to be released as an app for 5th-8th graders.
One other suggestion... talk to your colleagues about a Mix It UP At Lunch Day (these can be done on a regular basis, as in once a week). The idea is simple, break down the walls by challenging students (typical and atypical) to sit with and get to know someone they don't already know. Teaching Tolerance has resources for teachers about Mix It UP At Lunch Day http://www.tolerance.org/mix-it-up/get-started
Expand your concept of what you're teaching your students. Friendship skills is a big part of your curriculum. You're the leader. Go for it!
Are the lunches with a class or can each student go their own way? Or is there another student that you think this student will have common interests with?
I would start by giving him opportunities to meet some other students that he has common interests with and working out from there.
Does your school offer social skills group or anything that promotes all kids on the spectrum the ability to make friends?
We have something called lunch bunch. Once a week a handful of kids get to eat lunch in a room separate from the cafeteria. It is a mixed group of special needs and of "typical peers". It starts as a guided conversation and then the moderator backs off as the conversations become appropriate. All the typical parents have to sign permission slips to be part.
We also have social skill groups that play games 1x a month...I wish it was more often but I will take what is offered.
It is wonderful you are asking! Definitely makes you a very caring teacher to try to help this student make a friend.