My ex-spouse and I have joint custody. He pawns off the child rearing on his mom. Can I revise the custody arrangement?

That is not an unusual circumstance. I see that a lot. The answer is maybe. If, when you initially entered into your arrangement, it was understood that the was what was going to happen, then probably not. If that wasn’t the understanding of what was going to happen, then perhaps so. If you can convince the court that that is substantially different than the circumstance that occurred at the time you entered into your deal for child support or when the judge imposed child support on you, then you can usually trigger a rerunning of the guidelines with the number of overnights with each parent changing. You may also couple that with a change in time-sharing, that the child will spend not more time with the grandmother but more time with you.