Hi Sherry, it's good to hear from you!
I'm glad you and your husband have talked and both of you want to work on your marriage.
Although I can understand that your husband might be upset about your son being in the bed with you, he had other options about how to handle it. He could have come to you and talked about how he felt. Or he could have suggested therapy. But the fact that he chose to go outside of your marriage says that he is thinking of himself. By turning to someone else, he has added problems to your marriage instead of solving the original issue. Many people go through problems in their marriages and do not turn to other people.
Because of that, it sounds like a lot of this issue is about your husband more than it is about the marriage. If he feels as if he is having a mid life crisis, that could mean many things. But most men experience feeling of insecurity either from past issues like childhood abuse or it could be that he feels his life is half over and he has not accomplished what he wanted to by now.
It sounds like your husband is having a difficult time with knowing what he feels. Men often have affairs for a variety of reasons. Sex, power and narcissism are often reasons along with unresolved emotional issues. But they also do it because they can. Women are more likely to stay in a relationship where the husband is cheating because they are more emotionally invested in the marriage. And men sometimes can take advantage of this.
If your husband is willing to work on the marriage and has given up the other relationship, then the first step is to rebuild trust. If you can trust him again, it makes rebuilding your relationship a lot easier.
Also, he needs to start making you the center of his life. There is no way to regain a spark in the relationship if your husband is focused on himself. He needs to start refocusing on making you happy. That will help him feel happy.
You can also do the same. Put your husband first in your life. By doing this, he gets the attention he needs to feel he belongs.
Listen to each other. When you talk, each person gets 5 minutes to say what they need to. The other person's job is to listen then say back what they heard. Use "I" statements when you talk to each other. For example, when you talk with him say "I feel sad when you...". That way, you own the feeling and are not blaming each other, which can cause resentment.
Most of all, it is important that you both go to therapy. If that is not possible, your husband should go on his own. To find a therapist ask your doctor for a referral. Or if you attend church, your pastor may be able to help. Pastors are often good counselors. This will help you both learn to communicate your needs better to each other and have a safe place to air out your concerns with each other.
If you had spark in the beginning of your relationship, it will come back.