If there were fewer mice where he lived before, he will be enjoying himself thoroughly now.
Tell him firmly that he plays with mice outside! If he brings in one living mouse that is ONE TOO MANY.
By putting him and mousie out and locking the cat-flap you should manage to teach him that he stays outside with them.
Just make sure when you open the cat flap again, or stand at the door and call him in, that he comes without a mouse. Open the door just sufficiently for you to see whether he has a mouse or not in his mouth but not wide enough for him to come in.
My queenie cat started bringing live mice in, when her brother became allergic to their food! She then woke him and handed over the mouse with appropriate instructions.
Once he and I cottoned on to the need for a change of diet, she went back to bringing in dead mice when she wants to make me a present of them, and eating her mice outside.
Her dear brother on the other hand, eats his lunch on the indoor door mat if I am not at home, leaving the head and stomach of his mouse for me to clear away when I come home!
And yet we love our puddy-cats, don't we?