Get HTC Desire HD into recovery / fastboot / adb with dead volume keys

So my friend tried installing a custom rom on his phone resulting in bricking it. He gave it to me to try to fix it.

The phone is a HTC Desire HD with physically broken volume buttons, so I can't get into recovery . I can't tell what exactly he did because he doesn't know either, however the phone is stuck at the HTC bootlogo. Plugging the phone in into a computer shows it up as MTP device. I installed HTC drivers. Adb and fastboot are working too (using it for my own phones). However windows doesn't recognize the phone as something else. I already tried changing the driver and pointing it to adb and htc drivers but windows keeps saying it's a) the newest driver installed (MTP) or it's not compatible.

I tired getting into the bootloader by pressing the back button while powering on, but it just stays at the bootlogo. Plugging the phone into a computer while powered off shows it as MTP device too.

Any chances to recover the phone? Last way would be to replace mainboard I think, but the phone is to old to invest in it, isn't it?