Definitely a good chair is vital --- bad posture can lead to neck pain, back pain, headaches, etc... An adjustable desk is also ideal -- they have really reasonably priced manual ones that do the job nicely.
Working in IT, using multiple monitors is also pretty common. Usually would have email/office/web conference stuff on one and the coding/infrastructure stuff on the other (pretty much defacto now as to how we work in our office now.) As long as his device supports multiple monitors the dongles/connectors are pretty cheap.
Ergonomic keyboard and mouse also very important.
If he'll be on the phone a lot - a bluetooth headset is really helpful. during long meetings and/or if you need to be able to type or multi-task while working --- plus with the microphone being closer to your mouth you don't get as much background noise (dogs, doorbells, etc...) on calls.
Lighting is important since staring a screen while coding is really tough on the eyes. Blue light that most devices throw off but there are whole sites devoted to suggesting proper lighting. Lighting may be as simple as switching bulbs. If not possible then sometimes screen filters can help as well.
Speakers, a fan (for air circulation and white noise), a wall or desk clock (yeah I know they have clocks on phones and other devices but back in my coding days I would literally spend hours heads down with no sense of time at all. Having a physical clock was something I found helpful.)
Working at home as an IT professional due to COVID-19 I find I work mostly online (so while at the office I'd use a printer/paper, whiteboard, etc...) but there are a lot of online collaborative tools in play now (meeting, document collaboration, web conferencing, etc...) so some of the things you'd traditionally use aren't really as necessary.