I'm sure it's a mixture of a lot of things.
First, they lost their daughter...their grandson is a part of her and it's the only thing they have left of her. They want to keep that.
Second, they probably don't think you can sufficiently take care of your son given that you have college, full time, 2 jobs and a girlfriend.
Third, it probably is partially out of spite. You got their daughter pregnant and didn't stick around for her and your son...you're with a new girl and they most likely don't appreciate it.
Unless you're deemed an unfit father (not being able to provide for your son financially, not being able to provide a safe environment, having dangerous people around him), there's no chance they'll get custody of your son. Birth parents take precedence over grandparents. They might get court ordered visitation...otherwise, they're going to have to learn to play nice if they want to see their grandson. However, i don't recommend leaving your son alone with them until you get all of this settled.