Hi, thank you for your question. Based on the information you have provided he will be able to apply for divorce under unreasonable behaviour. He will need to complete a divorce petition and provide a £550 court fee to issue this. You state his financial position is not great, and therefore he would be able to apply for a possible fee remission using form EX160.
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The divorce will not deal with the child arrangement issues and this must be looked at and agreement attempted separately. In relation to child maintenance, if they both live in England or Wales, then the CSA/CMS has jurisdiction and child maintenance will be calculated based on the non-resident parent's gross income and how often the children stay with the non-resident parent on average. For his case it will only be in relation to the 5 year old as the other children are not his and it does not appear that he has assumed financial responsibility for them.
Both of you would be entitled to claim financial relief from the other arising out of the marriage .
Apologies, I was meant to say both of "them". So they can each seek financial relief from the other arising out of the marriage. You state that there are no assets, but financial relief can include spousal maintenance if they cannot meet their reasonable needs.