Legal Question in Family Law in California

what is your question?....is there a marriage or is this a paternity action?

You stated that there was "joint custody", but without a legal action, so its presumed that the custody agreement was just the two parents being amenable to each other.

Agreeing to private financial terms and conditions is not a very good idea when the other parent is holding visitation hostage, as the agreement may very well be enforceable later. California uses formulaic calculations for determining support. Your son should first seek a parentage action, unless he was married to the mother of your grandson, or in some other way is already the presumptive father. Once the parentage action is completed, an action for custody/visitation and support would normally follow. The calculation of the actual amount of support would be governed by the formula (Google "Dissomaster"). Also, I have seen these attempts where the custodial parent (mother in this case), attempts to set terms for "back" child support. In most cases, child support can only be back dated to the date of filing of an action for child support.

Given that you are even half stating the question, I presume that you're interested in possibly restarting visitation and therefore making some agreement for support. Seek out a competent family law attorney in your area and present the full facts to them. They can help you seek visitation or shared custody without it being held for ransom against you.