Hey @workworkworkugh I was going to suggest making his favourite slice or biscuits and adding them to his stuff for when he comes to collect it, but I see that has already happened. Keep it in mind if there is a next time.
I agree with the excellent advice @Sssloou and others have written. I know you have dismissed (for now) the idea of fun activities, but I agree they are key. Your family home needs to stop being the tense one, and return to be the homey, friendly, fun, family place in his heart. Can you do a movie night tomorrow night with your other boys? Favourite meal or takeaway and a movie night all under the same doona in the living room. Do your sons go to watch him play sport too? Depending on the time (and sport) could you take a picnic lunch for the family? Have a good time supporting him. If he comes over at the end tell him you are proud of him and love him, and then wave him off back to them, if necessary. (It might take a few games to build up to this). You could slip him the package of his favourite biscuits as you do.
Maybe in a week or fortnight or when things have de escalated you could invite them both over for a Friday night dinner - say you will make his favourite meal and her favourite dessert, if he tells you what it is. No begging to stay or come back, just an invite to dinner. He might say no. No matter. Ask again in a fortnight - maybe this time it’s a Sunday afternoon BBQ with his favourite cousins over to visit. After that you get tickets to a big sporting match for the same sport he plays (eg rugby - the Watatahs, or soccer or AFL etc) and say your family are going with his best friends (who also plays the sport) family. Does he and girlfriend want to come?
Yes or no, these breezy, fun invites will sound good, and as the drama with the girlfriend ramps up, start to look like pockets of peace to escape too. Always invite the girlfriend, so it’s not a competition. As @Ssslou says, give her the stage at these things. It’s not a competition. She will show herself to be what she is, and eventually, he’ll see it too. He just can’t as he is too emotional and caught up to see clearly now.
Win him back by showing and advertising the wonderful life he had. Make it even better. Thinking of getting a dog - get the dog! Always talked about a surfing trip to the untamed WA beaches? Plan it now. Install the pizza oven in your backyard and have pizza topping competitions with your kids every Sunday nights. Whatever works for your family. It will be nice for your other children to have some spoiling by doing this too, as they must be upset at seeing you so upset, and losing their brother in the house.
PS I know getting a dog is a big commitment. It was an example of possibilities. Obviously only to do if the OP’s circumstances are right.