Thank you for your question
My name is ***** ***** I shall do my best to help you
I am afraid that there is indeed nothing that you can do to force your ex to spend time with his child - you can write or you can get a solicitor to write - but it will make no difference.
You cannot make this man be a good father - he does not have it in him
I am afraid the only answer is for you to stop factoring this man in your lives at all
I am sorry I know that this is not what you wish to hear but sadly this is the legal position as it is
Please ask if you need further details
You can pay a solicitor to send such a letter - but your ex is free to ignore it rather than reply.
However by writing you are simply causing more grief to you and your son.
Your son is lucky - he has a great mum and that is worth much much more than money.