He just needs to sound interested but not share too much information about what his final decision will be or what other schools have told him or offered him. If he is receiving attention from scouts as well, he shouldn't tell them that he is 100% going to college because this will eliminate any chance of getting drafted. I know lots of guys that were being looked at out of high school but said they were going to school and never got drafted. Tell him to be honest but don't show his entire hand. If he tells anyone he has 100% made up his mind then has has a chance of getting manipulated - the school will offer less scholarship money if they think the athlete is 100% committed to going there, but they also might back off if they think he isn't really interested and has better offers. It's fine to let on that other schools are recruiting him as well, but there's no need to tell all. At the end of the day, his play and abilities will do the talking.
Those interviews don't really tell coaches much except whether the recruit is a total airhead (or the opposite end of the spectrum - exceptionally bright and/or driven) in addition to how serious they really are about playing at the school. No need to stress about saying the right thing because at the end of the day, his play on the field will do the talking.