Clearly you aren't in a relationship with your son's dad... since you mention having a boyfriend who is still legally married. And therein lies [just] one of the reasons you were refused a visitor's visa ...the second being that you're currently unemployed - albeit with good justification, i.e., you are caring full-time for your child - and receive maintenance payments from the boy's father.
Situations like these - in themselves - would be sufficient grounds to deny your application. But there is also a third probability ... which arises from the understandable suspicion that lurks in the mind of practically all Entry Clearance Officers - as a result of flagrant breaches of the System in the past - notably where visitors *fail to return to their own country within the timeframe allotted. I'm NOT saying you'd be likely to fall into this *category... but you can easily see WHY visitor visas are notoriously difficult to obtain.
So ...if I appear negative, but it'd be completely wrong of me to build up your hopes.