He may be or he may not. Since you are asking me to comfort you about his immaturity, it seems to me that he is immature. There seems to be a problem. I am an 18 year old guy myself and I know that if my mother was looking up this question online I definitely am in question. However, why does this matter? Is he in college, or is he goofing off? Is he in jail? Did he get someone pregnant? As an answerer I need to know more of what's happening.

What is he doing , or not doing that makes him immature?

Please tell us what he's doing that you are concerned about - and what he's doing right. It's hard to know whether he's immature or just taking a different path than you had in mind, or if he's generally immature or just immature about some things.

Heck I have four boys in their 30's and still worry about them.