It sounds like your boyfriend is stalling. First of all, you're already living together, so how is that any different than being married? It really isn't. The only thing missing is the piece of paper. In reality, you've already entered into a marriage relationship.
You didn't say what happened to his first marriage, or if his son lives with you both all of the time, or just on weekends, etc. That makes a big difference. You also didn't mention what kind of relationship you have with his son, which also is very important. Do you treat his son as your own? Have you bonded with his son? Perhaps he's basing his feelings on how well you get along with his son -or not.
Does his son have some kind of problem? Does he have ADHD, conduct disorder, autism, etc.? If so, then his concern that you have a better paying job with benefits might be understandable. Treating disorders can be expensive. Perhaps your husband is afraid of having another child with that same problem. Or maybe he's doubting his own ability to parent - or yours.
While money is definitely a consideration when thinking about starting a family, if everybody waited until they were "ready" financially, I suspect there'd be far fewer kids on the planet! No one is ever ready, in that respect!
You said you are in graduate school - how close are you to graduating? The good news here is that you will undoubtedly make more money with a graduate degree, and you will also have more opportunity to get a good job. This works in your favor even if you should opt out of your current relationship, so in a sense, it really is a win-win situation for you.
If, after getting your degree and better job, he still balks, then I'd say cut your losses and use your newly acquired educational status to get a new life -and maybe a new boyfriend. As you said, your biological clock is ticking. But whatever you do, don't try to combine graduate school with having a baby. I can promise you that while some people do it, it takes alot of support from their significant others - and it doesn't really sound like you'd have that.
Good luck!