I think a soundbar would be helpful here. See if you can find a cheap-ish one with a physical volume knob.

Plug the TV into the soundbar, and turn the TV volume all the way up. With the TV at max volume, adjust the soundbar volume to whatever maximum level you find acceptable. Super glue the volume knob in place so he can’t make it louder. He’ll still be able to adjust the volume up and down on his TV, but it just won’t get any louder than the volume you set on the soundbar. If you think he might unplug the soundbar to get around this, then you can also superglue the cable into the TV and soundbar. Just make sure the glue doesn’t touch any metal part of the cable or the ports that it goes into. You’d really only need a drop or two of it anyway, so it should be easy to be careful about it. I’m not sure if superglue is conductive or not, but if it is, it could cause a short and make things malfunction if it gets on the metal bits.

I realize this solution is far from elegant, but I can’t think of a better way that doesn’t involve taking the TV apart and disconnecting the speakers or adding resistors, or what have you. Just keep in mind doing this will void your warranty on the TV if you have one!

I realize this is a bit of an odd one, but I looove the menu music in Sonic Mega Collection Plus!

Why is this relevant?