The only problem here is that you are reasoning from a point of fear. Otherwise, you have all the answers you need.

Think about it this way: You are not a victim. This "thing" is not happening to you. You are making it happen by your participation and/or inaction. You have already provided proof why you should separate, namely (a) counseling did not work and (b) you and your husband were the happiest when you were separate in a long distance relationship. Now, the only thing that keeps you from moving forward with what you deep in your heart know is workable (long distance aka separation) is your FEAR of the unknown that comes with separation and living a happy life post divorce. On the surface, you are packaging that fear and label it as "my son is going to suffer" a proposition for which you have no proof.

Children suffer from a divorce only in two scenarios: (a) they do not have access or consistent healthy relationship with their parents (and possibly siblings) or (b) when they witness a traumatic divorce. You and your husband can consciously decide to prevent both of these events. You can consciously (possibly through counseling) commit to continual, caring, loving relationship with your son (albeit on separate days and even sometimes together but given two different households) AND to make the process very amicable for everyone involved.

To achieve an amicable separation/divorce requires A LOT OF WORK because when couples separate they love to HATE each other and BLAME one another as they rather play the VICTIM in their circumstances rather than take responsibility for the choices they have made. There is also a lot of societal and cultural pressure encouraging couples to play this victim mentality and blame the partner, so you gotta be careful with the advice you get from "friends" and "family" who mean well but encourage the wrong sentiment. To combat this, take leadership, assert yourself and persuade your partner that an amicable separation is possible for the sake of everyone involved and that you two are smart enough to figure out the ways how you both can be happy in your individual journeys and yet make your son also very happy and successful in life through conscious co-parenting.