You're making perfect sense, and I empathize completely. I've blogged extensively about my secondary relationship with a married woman on my blog -- http://www.polyamory.com/forum/showthread.php?t=4540
-- perhaps something somewhere in there will strike a chord for you.
6 years is a long time. I think searching for another bf is a really good idea. Maybe a new partnership would also stay secondary, maybe it would eventually become primary, maybe you'll find a place of comfort with a few "lover-friends", no way to tell, really. I would hazard to say that finding someone new probably won't make you long for more time/commitment with your current bf less... but that it will make it easier for you to take a deep breath and focus on something else (or, more accurately, someone else) without dwelling. Plus it can bring another person into your life for support and for that feeling of family, which can be so important to overall well-being.
Its really a shame about your bf's wife's refusal to engage with you.
Being in a secondary relationship can still mean lots of satisfying involvement in your partner's life... but not if their other partner doesn't want you there. My condolences on that. I know you probably can't change that aspect of the situation, but do you know why she has that no kids rule? Is she afraid this is going to harm your son in some way (many anecdotes, and, recently, an actual study, disprove this idea)?