Preface: I am not fully aware of Auburn's policies or the general climate of their IFC.
Just to let you know two things:
1- Fraternities don't have quotas (with rare exceptions at a couple of schools). We are allowed to send out as many bids and take as many pledges as we would like.
2- Many schools have the formal/semi-formal rush period, and then an informal time period after rush when they can still give out bids.
I'm sorry your son didn't get a bid. One factor about IFC rush is, there will be another rush in Spring semester. Until then, your son can try to get involved with other activities on campus that will get his name known to others. Also, if he is friendly (genuine, not stalker friendly
)to some of the other guys in his classes, he will get to know them and they will get to know him. He will hear and possibly get invited to some parties.
Also, he should go to more than just 1 group's parties (I'm sure he realized it by now). A group he may not think much of right now may end up making a great impression on him later.