Thanks for the info. So your sex life dwindled after you had your son - basically, for the entire length of your marriage? Ouch :(
Having a small child is incredibly wearing. They make constant, urgent emotional demands as well as all the physical ones such as feeding, play, exercise, education, bathing and dressing. The vast majority of women go off sex for a couple of years, simply because they've no energy left for it - and that's even when they have a partner who really does his share. At least, in that case, they're both too tired for sex 
Four/five years is quite a long time to be feeling like that, though. How hands-on are you around the house and baby? The true test of this is: How much time do you spend watching TV, playing a game, dozing and stuff (leisure) compared to her? Is she always 'doing' even when you've switched off?
It's a bit difficult in your situation, because you didn't have the time to form a solid relationship before forming your family. It's possible that you're incompatible in the long run. First, though, I'd look at the obvious which is how the energies are spent in your household.