For a child that is 6 years old, a judge might decide that alternating full weekends with each parent is better for the child than one night each weekend with each parent.
It depends on the particular facts / circumstances.
(NB - obviously I don't know the particular facts, so my points below are really just thoughts/ideas)
Your post makes clear why your DB thinks that one night each weekend is a better idea. But what reasons does, or could, DN's mum give for alternate weekends being a good idea? (also consider what DN's mum's lawyer might say)
For example maybe she thinks that it would be good for DN to have a regular Saturday night with her? clearly Saturday nights are nicer for a 6 yo child than Friday nights when they are worn out by the school week. They are also nicer than Sunday nights when they need to get ready and go to bed early for school. Maybe she thinks 2 consecutive nights each fortnight with dad is less disruptive for DN than 1 night every week? (this can be the case for children that find family transitions taxing or if there is a commute involved between mum and dad's place); maybe she thinks it would be easier for her to plan pleasant weekends for her and DN if they have 2 days/nights to flex around; maybe she would like to take a weekend trip now and then (this is in DN's interests if it leaves her more rested). Are there any step-siblings or relatives she is trying to co-ordinate with for DN's benefit?; does she need a full weekend for work or personal reasons (after all a happy, earning, and/or fulfilled primary carer is in DN's interests).
You say that DB cannot get home earlier than 8.30pm any day of the week. I can appreciate there may be a variety of reasons for this (some of them quite understandable), but it does indicate that he prioritises his work ahead of his 6 year old child. In contrast DN's mother might be prioritizing DN many nights of the week, even though it affects the jobs she is eligible for and her earning potential. If this is the situation, what DN's mum says about the child's best interests might be seen as more credible than what your DB says.
Have they tried mediation?
Maybe if they talk it out they could agree
Week 1 - shared weekend (friday night with mum)
Week 2 - shared weekend (friday night with dad)
Week 3 - weekend with mum
Week 4 - weekend with dad
Added to this - it could help to have an agreed date to review in say 4 or 6 months time (or earlier by agreement) so that neither feels unduly committed to a compromise that neither of them really wants.
It would also be sensible to ad an agreed time/s for Skype/facetime so that son and father see each other at least once per week.
Dad could point out that he is prepared to go to court to ensure he sees son regularly. Fair enough, but Mum could point out that if he's not prepared to adjust his working hours during the week, then he doesn't have a strong case for calling the shots on the weekends. Dad might point out that working long hours during the week helps the the child because he pays good child maintenance. (Of course if he has been dodging or paying very little child maintenance, he can't say this).
In the longer term, if Dad does adjust his working hours, they could then transition the above pattern straightforwardly to:
Week 1 - weekend with mum
Week 2 - weekend with dad
Week 3 - weekend with mum
Week 4 - weekend with dad
Plus weeknight/s seeing dad or staying over at his place, every week or if it suits the child better weeks 1 and 3.