Some suggestions: Call around to see what day care/preschools would charge you if you went back to work full-time. (While you're at it, find out how much they would charge to do the amount of baby-sitting you do.)
Then figure out if you could find a job that would cover the loss of your babysitting income and your husband's part-time job's income, day care/preschool for your son, transportation costs (more gas and wear and tear on your car), work clothes, higher meal costs (you might rely on convenience foods more often than you do now) and paying higher taxes because your job would take you and your husband into a higher tax bracket.
Unless you can get a really good job, you might find out that you can't make enough to cover all that and that it wouldn't pay to go back to work full-time. (You didn't say what your earning capacity might be.)
Would your husband be able to watch your son if you found a part-time job where you only work nights or weekends? Or would that be too much stress on your relationship?
Speaking of stress, you do realize that the stress you feel about money now might not be anything compared to what you would feel if you worked full-time!
You only have a couple of years till your son is in school all day and you could work while he's at school. That's one thing to consider. Could you make it for another couple of years?
A couple of other possible solutions: find a job where you can work from home, either for someone else or going into business for yourself. The job market isn't great, but you might have some marketable skills that you could use in this way.
You could also consider taking in one more child to babysit. I realize that would make things crazier, but sometimes it's not much worse than having one (plus yours of course). You don't say whether you get paid under the table. If you are, you could point out to the woman you babysit for that she's really getting off easy. If she didn't have you, she'd probably pay another babysitter or day care more plus she'd have the hassle of having to do all the paperwork (as would you) if you got paid according to tax laws. Also point out to her that she could claim a child care deduction if you were above the table. (Check the latest tax rules to corroborate this--I'm not a tax expert.)
Sorry for such a long answer. I babysat when my children were young and it wasn't easy (or lucrative). But when they were very little it was worth it. I was able to stay home till my youngest was in school. (I had four.) Then I went to work full-time and to be honest I hated it. But then I didn't have the best job in the world. (I worked for the post office.)
Make up a list of pros and cons and see which outweighs the other!
I wish you the very best.