Hi toody, hard luck, it sounds like a miserable position to be in. I'm a family solicitor and am happy to give you the following thoughts that spring to my mind.
First a question: how old's your son?
I assume you don't have any formal agreement or court order about your ex's contact with your son in place at the moment?
If you have a formal agreement or a court order, then no he can't just suddenly start demanding to see your son more. If he wants to vary an existing agreement or order, he has to go through the proper channels (i.e. your solicitor or apply to court).
If there's no agreement or Court order in place, then it's up to you and your ex to try to agree contact. If you can't agree and you don't want to him to have as much contact as he wants, then he'd have to apply to Court to get a court order for contact. The Court would firstly try to help the two of you to agree contact arrangements, but if you can't, there would eventually be a hearing to decide what contact your ex should have, based on what the Court considers to be in your son's best interests, having heard evidence from both of you. The history of his contact so far and his relationship with your son would be relevant e.g. if he's historically not shown any interest and only popped over for an afternoon every few weeks, the Court isn't going to suddenly decide he should see your son for two nights every other weekend, although they might decide a slow increase in contact is appropriate.
I think it's completely reasonable to tell him he can only text you once a day (I'm assuming your son is too young to speak to him on the phone or text himself?) and to ask him to keep any abuse or personal comments out of the texts. You could confirm that if he ever needs to know anything about your son over and above the one text, you will contact him if necessary. In the meantime, I strongly suggest you keep all the texts he sends you, as they could prove very valuable evidence of his behaviour and attitude towards you if this did ever go to Court - even if it didn't, a solicitor may well want to refer to them in a strong letter to your ex.
Hope that's of some help and good luck.