The mother of the groom does not have as many responsibilities as the mother of the bride. She certainly does not have to worry about things such as the wedding budget or finding the perfect wedding gown. Even so, there are still some things that she should be doing to help out.
The first thing you should do is contact the mother of the bride and introduce yourself, if you two do not already know each other. It would be a nice gesture to get together for lunch or dinner so the two of you can get to know each other.
Your guest list should be sent to the mother of the bride as soon as possible. Double check the list to make sure it is complete and accurate. If you had previously talked about limitations on the number of guests make sure your list conforms to those limits. If for some reason you cannot make the list that small you should politely bring it up and see if any compromise can be made.
As the mother of the groom it is your responsibility to make hotel reservations for any out of town guests which you invited. You can contact the mother of the bride and see if there is a specific hotel that she is using and you can register all of your guests at one central location.
The biggest responsibility for the mother of the groom is throwing the rehearsal dinner. A rehearsal dinner does not have to be formal or fancy, but it can be. If you want something a little more casual you can throw a barbeque at your house. When inviting people to the rehearsal dinner you should remember to invite everyone who is in the ceremony including their spouses. If there are any children in the wedding such as a flower girl or a ring bearer, they should also be invited along with their parents.
Even though there is not a lot of things that the mother of the groom absolutely has to do or is expected to do, if you really want to help more you can always offer you help. If you are already close to the bride or her family you can ask if there is anything you can do. If you know a good baker or florist you think they should check out let them know, just make sure you emphasize the decision is up to them. Finally ask your son what he wants you to do. Men are helping more and more in wedding planning than ever before. If he is helping in the planning he may want your assistance. Even if he is not helping a lot he probably still needs you. You are his mother and the engagement and wedding planning stage can be stressful for men, he may even need you just to explain what his fiancé is talking about or why every little detail is so important to her.