Hullo there! We are going through a bit similar situation with my partner, so I thought to chime in. I leave the salve and vitamin tips to the more experienced among us. The only thing that would come to me on top of my head would be a good warmup and avoiding hard individual strikes, but if your bf is as experienced as you say he is, he should be realizing this too. And bruising is unpredictable, as you said - no sure way to guard completely against it.
My partner looks like a mess after this weekend's play party. She's seen a lot of things in the scene and her bruising right now beats (sorry for the pun
) everything she's seen before. It makes me physically nauseous to see.
We've talked about her and new bf putting aside certain instruments for a while. She's a masochist so pain is her poison and it's very hard for her (from my POV) to draw boundaries around that. We've agreed for her to take a month off all pain-inducing activities and to really gauge her feelings and limits around the issue. While your bruising certainly doesn't sound like something that could leave permanent scars/marks, maybe taking a set-date break from certain bruising activities could work?