The stuff that actually matters for a gaming setup falls into a few categories beyond the flashy peripherals. A quality monitor arm or desk mount frees up space and lets him adjust height and angle, which matters way more than people think once you start spending hours at a desk. Similarly, a good desk pad - the large kind that covers most of the surface - does wonders for cable management and aesthetics while being genuinely functional. If his cable game isn't already tight, velcro cable ties and some decent cable sleeves or channels make a real difference and cost almost nothing. He'll probably appreciate these more than another RGB thing.
The second tier is comfort and air quality. A decent chair cushion or lumbar support pillow if his chair isn't top-tier, or even something like a small desk fan for airflow during long sessions. Some people overlook this stuff but it genuinely affects how long you can sit comfortably. A monitor light bar is another solid option - reduces eye strain and looks clean without being unnecessary RGB clutter.
If you want something a bit more premium, consider a nice surge protector or power strip with USB ports specifically designed for setups, or even a KVM switch if he's got multiple devices. These feel like "boring" gifts until someone actually has one and realizes they solve real problems. The key is that he'll get more mileage from something that makes his setup function better than from another decorative piece, even if the decorative stuff is fun to buy.