Me and my partner split up and we both agreed my 6 year old son would live with me now after 6 weeks she wants him back he is in a new school now with new friends and is very happy and has made it clear he wants to stay with me and not go back to his mum

It is foolish to make arrangements for custody and/or visitation outside of a Court Order. I just posted an experience I have had with a similar situation - https://www.askmehelpdesk.com/family-law/parental-interference-kidnapping-601764.html

The Court will not presume that the child knows what is best for him.

Go to Court and have a Judge make a determination.

... he is in a new school now with new friends ...

When you go to court (and, as JudyKayTee suggests, it is important to do that ASAP), it won't look good if you changed schools in mid-term (if that's what you did). But it's "water under the bridge" (i.e.: a fait accompli) now, I suppose.

By your use of the term "mum", am I correct in assuming you are in the UK?