I suggest you play some games with him. Try "Simon Says" using words and then do it with gestures. Use a symbol for Simon Says (e.g., pointing to your mouth), then point to him (or another child) and demonstrate the behavior. This is fun and will also allow you to find out if your son can respond more quickly to visual cues (like hand signals and pantomime) than he reacts to language. If you aren't sure, then have a speech and language pathologist/therapist take a look at him. Ask the school if one of their staff members can observe/evaluate him.
You say he might be lacking in social skills and the teachers are concerned about his maturity level. I'd ask, compared to whom? If he's being compared to the other kids who are going into kindergarten, some of whom may be almost a year older than he, then this is an inappropriate comparison. Ask the kindergarten teacher to observe him, or pay a visit to a developmental psychologist who has a lot of experience with little kids.
Finally, do I think it's ADHD? There's no way to know without more information and further observation, but wouldn't it be a shame if this were the conclusion, when the only thing that's "wrong" is that he's just a little kid? Take your time, ask a local expert, and don't jump (or let them jump) to conclusions.