“What to say to someone who lost a parent? I’m not sure what to say/do/where to start with my partner’s loss of his mother recently but i want to be there for him”

I am very sorry to hear about your partner’s loss. And I think you already have answered your question. There is not much you need to say. You just need to be there. Listen to your partner. Be there. Show him love by preparing his favourite lunch, buying his favourite treat, and doing the things he enjoys.
Nothing you say will bring his mother back. Listening, being there, showing you care, and you understand his pain is the best thing you can do to support him at this time.
I wish you both all the very best at this difficult time.
Love,
Ana