need advise

So I ebf and I'm having trouble with production .. I think. This may be a long dragged on story so sorry.. bare with me. He hasn't had formula since he wa 3 weeks old and I'm very proud of that. He's now 4m. I have been back to work and pumping while at work every 3 hrs and I'm getting anywhere from 5-7 oz when pumping each time.. well his grandmother who is keeping him is saying he's taking 8oz at a time .. every 4 hrs.. I have asked her to try and cut back to 6.. bc 8 is a lot ( and bc he weighs 19lb 13oz) well she said he's not having it.. well I am running out of milk for her so literally I'm pumping today for tomorrow. No milk saved for if we wanted to go do adult stuff on the weekends... I'm trying to pump other breast while he's nursing but then I end up having to give it to him bc he's still hungry after feeding on one breast. I just can't keep up. He's kinda cluster feeding right now in the middle of the night.. I'm just stressed. I want to breast feed until at least 6m but I'm afraid I'm gunna have to start supplementing.. and I know it's not a big deal to give him formula but I have been so proud of it. And it's one things I feel like I'm doing right but lately I feel like I just can't keep up... what should I do?! If I do start supplementing some and pumping more what is the best formula to give him?! I don't wanna hurt his tummy.. I'm stressed thinking about it ladies. I think I enjoy breast feeding more than him. Love the bonding time. 😭😭😭😭