S.,
You show alot of maturity for not wanting to badmouth your son's father. Good for you.
To be completely honest, Julien's father being inconsistently involved in his life will be less damaging than having his mother (and/or father) try to discredit the other parent. So, as hard as it is, holding your tongue is actually loving your child for both of you.
I did this for 14 years with my daughter and she is now 16 and able to negotiate a relationship with her father that works for both of them. If you asked her, she would say she didn't miss out on anything because she had another full life aside from what was missing with him. However, her longing for closeness with him was always still there. I am very proud of her for forgiving her dad for mistakes he made in the past and, actually, helping him forgive himself. One of the main reasons dad's don't get involved in their children's lives are because they know they have screwed up, maybe not been there or whatever. And, we all know it's true - sometimes mothers don't really make it the easist thing in the world for these dads to try to do better. Why would anyone put up with all that hostility? Mothers who do that are selfish and controlling. As long as the child is not being abused, neglected, put in unsafe situations, get over it. I would ask mothers who berate their children's fathers to really look at why they are doing it. I strongly doubt, if they are honest and self-aware enough, that they are doing it for the children. Especially if they are doing it IN FRONT OF the children.
There is no reason why you can't just tell your son that you don't know why his daddy doesn't come around. You don't have to make anything up or say he has different priorities or is busy (how is THAT not badmouthing, by the way?). His father is responsible for answering those questions, not you. As he gets older, you can encourage him to ask his father.
It's alot of work for you but you know that being a single mother is hard work - I remember it well. I am married now and my daughter and I have a wonderful stepdad-husband/life but we remember fondly those days when it was just us in the apartment freezing and not having money to ever go anywhere!
It can be OK if you can just stick with the long-term idea that not badmouthing this guy is better for your son over his entire life.
Good luck to you!!!