Rewarming leftover breast milk

I’m a FTM and see different things on the internet about bottle feeding with breast milk and warming it up.

I made a bottle for my son with frozen breast milk (4 oz that I warmed under hot water) and he only ate 2 oz. I put the remaining 2 oz back in the fridge. Can I re warm the 2 oz so he doesn’t have to drink it cold? Or can I only warm my frozen milk just one time. Thank you!