A promise to do something - even if it's in writing - isn't enforceable unless there is some "consideration" (e.g., what is he getting in exchange for the support he sends you?). Without consideration, his offer is just a gratuitous promise without any real obligation on his part. If he is serious about providing support, I suggest consulting an attorney in your area to draft an agreement that includes all of the elements needed to make it enforceable and the parties' intentions clear.
This response is for general informational and educational purposes only and does not constitute specific legal advice nor establish an attorney-client relationship. For specific legal advice you should consult a qualified attorney in your jurisdiction.
You would need a local lawyer to draft your agreement for you, as the other lawyer first pointed out. Best of luck to you and your ex boyfriend.
Answer given for general advice and is not a legal opinion, which would require an analysis of the facts and circumstances as well as the applicable law and regulations.
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