Moving away with kids?

Have recently split from my long term boyfriend. We've lived where we are now for 10+ years but now that we've split up he's saying that he wants to take the kids (we have four, youngest is two) and move back to where he's originally from which is 5hrs away. His mum would often suggest we move there while we were together.

The kids have lived here there whole lives, have schools here, friends ect so assuming a court would recommend they stay here? Also he never spent any time with the kids while we were together, I did all of the childcare so I'm a little confused why he wants them full time. Happy to co-parent 50/50 but what can I do to prevent him from this?