Lots of choices but I’d probably use Kubuntu if your boyfriend is new to Linux and you want this “official” Proton support (not sure that actually means much; Proton works very well on most distros). The plasma interface can be set fairly similar to windows for a newbie to feel comfortable.

It’s all just personal preference of course; I just find the Ubuntu interface annoying as someone who uses Linux and windows a lot. Personally I use Mint; very nice distro, good and stable, nice for newbies, and the default cinnamon interface is very windows like too.