As previously mentioned, implement QoS and/or consider a second ISP connection or upgrade current plan (speed)…YouTube + Discord shouldn’t consume that much bandwidth but that depends on what your current plan allows.

Enable QOS on your router for your machine and see if that helps.

I think you first need to see what the actual full speed (the bandwidth) you can get from your ISP is, and compare that to what is being used by you and him.

I don’t know the tools that can show you that though, sadly, as I’ve never faced issues like that.

If it’s simply that your using all the bandwidth your ISP is giving you, then there really isn’t a whole lot you can do. Enabling QOS, as another user said, can help to balance the bandwidth, but that will just mean that you and him will (still) have crappy service.

However, if it turns out your household is not using all the bandwidth the ISP can provide, then you can start to pinpoint what the limitation is. Most home routes are not really that powerful, and thus can slow things down. But that usually only happens with more devices, not just someone using a lot of traffic from one or 2 devices.

Upgrading the router to a WiFi 6 or something better than what you currently have might help as well.

Better placement of the router for optimal signal could also be something to look into. This honestly would be my first step depending on your home configuration.

Another thought is going wired, if you aren’t already, as I’ve been assuming so far that your using WiFi. Wired connections do remove a little overhead, but in most cases it’s such a low hit that users don’t notice. But if you are hitting the bandwidth limit of your ISP, or your router, that could be a way to try to optimize things with the hardware you have already.

If you REALLY wanted to go all out, and make sure it’s not something on your internal network, you can build your own router, as in using an old computer. Opnsense is very popular in that area. I’ve never used something like that personally, but I’m gearing up to. Routers are just purpose built computers, that usually have low end hardware to do so many things. Building your own could mean any possible bottlenecks due to the router could be eliminated (depending on what hardware you have, and how far you want to take that). I don’t know your experience level, but id only advise that if you are REALLY comfortable experimenting and have some money to burn on the project. Otherwise I’d advise to use all the other methods before this one.

no idea what the current plan / speeds are?

if its broadband, and there is an option to upgrade to fibre and speeds.

also like others have said, check out the qos settings if it doesnt have, then may need to buy a router with qos features.