I don't think it matters at ALL. They can tell you anything they want to try to get you to incriminate yourself or help them get someone else. Remember now, cops WILL say anything in order to try to do all of that and more.
I could not agree more with the person above who said your son needs to remain silent. He does not have to talk to them. Even if they want to bring him in for questioning, without an arrest warrant, they cannot even do that. And if they do that, yes, you do need to get an attorney. You should definitely talk to one now, anyway.
They have nothing right now, or they'd have arrested your son. A text message can incriminate someone, yes, but it doesn't look like they got enough to do that. They're probably just trying to scare him, and it's working.
Remember, cops will not make friendly talk, and with something like this, they are never on your side.
Tell him to be quiet but polite with them, and talk to an attorney as soon as you can .
And talk to your son about selling pot, period, especially to minors. This should have been a close enough call to teach him. I hope he got hte message loud and clear.
I'm not trying to talk down to you, btw. I've just read enough about this stuff, specifically pot, to know, or pretty close to knowing for sure, that if they had enough to arrest him, they would have. And THEN, when they've got real criminal charges pending against him, they could make a deal: snitch or your charges will stick; cooperate with us, and we'll make it disappear.
And the person above who said that snitches get hurt? They're right. Be quiet, be polite to the police, but don't give them anything else.