All the medications you mentioned are benzodiazepines, the main difference is the half-life (the amount of time it takes for half of the medication to be metabolized) and perhaps to a lesser extent onset of action.
Benzodiazepines are commonly used for anxiety, insomnia and seizures. Somnolence is common with these types of drugs, but sometimes will decrease after you have been on them for a period of time. The other options are SSRI's, TCA's and some MAOI's. I am not a fan of most of the psychotropic drugs as they can have really nasty side effects and are often not effective for everyone.
Here is a breakdown of the benzos you mentioned:
Xanax (alprazolam): C-max (peak concentration) occurs 1-2 hours after administration. T1/2 (half-life) averages to 11.2 hours.
Ativan (lorazepam): C-max occurs about 2 hours after administration and its T1/2 is about 12 hours. It does have an active metabolite which has a T1/2 of 18 hours.
Klonopin (Clonazepam): C-max occurs 1-4 hours after administration and T1/2 is about 30-40 hours. This might be a good bet as he could take them at night and they would help for sleep and then would still be active the next day to ward off anxiety attacks. The only down side might be daytime somnolence since the medication would still be active.
Keep in mind that people with CF often have increased metabolism on the P450 cytochrome (which metabolizes benzos), meaning that the T1/2 might be reduced and the dose might need to be higher than one would expect for a regular person. Also, there are sometimes oral (sublingual) preparations of these medications which have a more rapid onset, which can be useful if he's having an attack.