J.N.
I would say give it time. Contrary to the beliefs of modern parenting, self-soothing at 5 months is not a normal thing for babies; and you can't necessarily teach it either (you've seen how well that works).
I'm going to get flak for this, but have you ever thought of having him sleep with you? We did this with our kids, mostly because I was nursing and because I was soooo tired (esp with the 3rd one!). They slept next to me for a good deal of the first year, although we did have a crib that we put them in when we needed the bed to ourselves for a bit, for whatever reason. I think we all slept better because of it. (Now, they are 12, 10, 7, and great sleepers on their own, go to bed in their own bed and only the youngest rarely even come into our room in the night, unless they are sick).