Ex boyfriend bought me baby gifts for the baby I'm expecting which he knows isn't his. We have just split up and he is now telling me he wants me to pay half towards the presents

What he gave you is yours.
When I give someone something, I never expect it back.
I have known weasels that may give something, then take it back.

They are gifts, not loans. They are yours.

On the other hand - give them back and get him and the stress of all the arguing out of your life.