Take a DAT course. A good one. Apply early. Don't be a dork on the interview (that was a hard one for me). Interview is really low key and chill don't stress over it.
Dental school is not that bad. I actually really enjoyed it.
Most importantly is understanding the field and why you want to go into it.
It's a very difficult profession. Mentally and physically.
Hopefully he has spent a lot of time working or shadowing in different offices. It is best to work in an office to see the behind the scenes. See as many different offices as possible. Each office will have its own philosophies and personality. See what style fits you.