They should get Saturday night and part of Sunday off starting in the third week. That being if they checked in on a Sunday, it would be 20 days from check in day.
Pretty much everybody will get it. There will be some limitations. People on light duty will get time off, but will be resticted to base. Same goes for those on company probation and duty platoon for the day. People on battalion probation will be restricted to the immediate OCS training area. Otherwise, if you're not on probation or light duty and don't have duty platoon or fire watch you're free to come and go until it expires.
Just taking a guess here, you're expecting to hear from your son. Everybody will have the opportunity to use a phone during liberty, even if they're resticted to the OCS area. Some platoons get done early in the afternoon, some go later into the evening before they get cut loose. Also if he doesn't have a cell phone there may be a big line for the pay phones or a wait to get checked into a hotel and use the phone. Either way, hope he's doing good.