I want to say that I don't get the idea you are trying to meddle in your son's relationship. I respect you for that. Some may suspect a triangle has developed. From watching people here and there in my life, I have this to say: I think it isn't fair for anyone but your son to judge if his ladylove has found another. For someone else to do so would be jumping to conclusions that may or may not be true. Let that be for him to decide to look into or inquire about or not. He needs wisdom, patience, love and support from you and anyone he might confide in or turn to for comfort. She deserves the benefit of the doubt. It could complicate things and cause problems and hurt, to worry otherwise, since it is your son's love relationship. He deserve to find out what's going on for himself, without people from the outside stepping in.
I've seen well meaning people: friends and family, lose friends and ruin relationships trying to interfere when there is suspected or even confirmed cheating. I've always thought it best if the couple deal with it themselves. I believe the outcome is better if an involved partner sincerely asks, if there is a strong suspicion of cheating.