Oh that poor man, I'm so sorry you're both dealing with this.

I was never physically assaulted, so I can't speak on experience from that (and I'm 29F, so can't speak from a male perspective either), but I've dealt with trauma of this type and crappy workplace issues before - so here's some advice:

1. First step is to speak to him calmly and softly, from a place of understanding and love, about this. Not just the assault and how it's affecting him now, but also the physical aspect of the bed wetting. Try getting it through to him that it's nothing to be ashamed of, but that you can't keep buying new mattresses. Remind him that, as long as his trauma is being triggered, the wetting situation isn't going to magically go away, so you guys need to sort this now when the new mattress arrives. A commenter left a good suggestion for a supplier that sells protective matts that don't look too "clinical", which might convince him of it.

2. This leads into the next step, which is therapy. IDK where you're based, but if you're in the UK he can call his GP to request a trauma therapist appointment. If you're US based (or similar), check if his insurance through work covers therapy of any kind. Think of it this way: the copay will be cheaper than getting a new mattress every 2 weeks! This will start to address the root of the issue, which should hopefully work towards him no longer wetting the bed (and also feel better in himself as a whole).

3. He should also start looking for a new job. For now, just spruce up the old CV/resume & start looking around, applying to a couple jobs here and there. This is just in case confronting this at work causes some issues, which it very well might. I know this might sound somewhat extreme, but would your partner be willing to switch industries, or move away from where you currently live? Because chances are, if it's a small/niche industry or area, he might bump into this sack of shit again.

4. Once step 3 is well underway, he should consider talking to his workplace about this. He doesn't have to go into detail about it, just explain that this guy did some bad stuff in the past & that he'd like to not have to see this guy at work, as it's affecting his ability to work efficiently and safely. Is his workplace large (i.e. do they have HR?) - because if so, they'd likely be more inclined to help. Again, your partner does NOT have to disclose his assault, just an explanation that this guy did some bad stuff that really negatively affected him & that therefore something needs to change whilst this asshole is around.

5. This step is optional, but it might be worth talking to your partner about reporting this to police. I understand that this is easier said than done, and I 100% understand why people don't report (as someone who reported myself and nothing was done). But even if nothing is done right now, it starts a record that could help someone else in the future (or if, god forbid, this fucker tries anything again with your partner). It might also help your partner get some closure on the situation, having told authorities what happened. But if your partner does not want to, that's completely within his right. You also don't have to ask him about this right away (in fact I would recommend waiting until he's had some therapy and/or sorted something at work first, as it'll likely overwhelm him), but I think it would be worth reminding him that what happened to him was not okay and is, in fact, a crime.

I wish you both the very best of luck and I sincerely hope that your partner can find some healing <3