They will give preference to your child . Wait for few months if you are satisfied with the current nursery
You can look for alternative for sure but usually the existing child is always preferred and drop outs are common too.
Take the extra day for now so you don’t lose it. I’d start looking for alternatives in the meantime. I know that my nursery offers existing children first refusal over hours rather than taking new children on. Ask if there is a list you can be added to to claim the extra day.
In a similar situation, someone else's circumstances changed and we were able to maintain our space. So much can happen between now and then!
Take the extra day for now. See what happens. You can always leave him in for the three days and find an alternative one for one day (lots of friends of mine do that. They find a forest school outdoors one tend to be best for a one day)
I would start looking for alternative childcare options, such as a different nursery or see if a family member can have him 1 day a week x