I also have to disagree with the others about the PI. In the end, either you will waste money and you learn nothing new or you learn something new and all it does is hurt you. Decide if an affair is a hard line for you. If not, don't bother. If it is and you just can't rest not knowing go with the PI. Although, it sounds like you do not intend to change your tactics either way so I don't see how it would be beneficial.
Just keep GAL! If you really suspect drinking obviously he is using it to cope in some way. You might just keep an eye on it. Then is there anyone else you can ask to see if they have noticed changes that would make people think he was drinking. Often alcoholics drink at lunch as well. That might be one way to find out. If you think he would agree ask him to a lunch date near his workplace. Obviously not if he would consider this chasing. Perhaps if you do not initiate contact while he is gone he would be more open to that suggestion when he gets back.
