Every time I go to dismiss him, the only thing he'll 'talk' about is whether or not I think we're ready t take the Castle.

PLEASE tell me there is a way out of this without loading an hour ago save

Choose another Companion. When he/she/it is activated, an settlement location table appears, asking where the previous Companion should go until you have need of them again.

Couldn't choose another companion as I "already have one".

In the end reloaded an old save

i have the oppisite problem. i can no longer recruit him. for some reason after i took the castle he was no longer a companion

I believe you would've had to complete the Castle quest before being able to dismiss him, hope you didn't have to travel back too far.

Quicksaves are lifesavers, do it often!