Unless he is in a sauna, it shouldn't matter. Phone isn't even charging and brightness doesn't seem to be that high. My S20 would ONLY reach these temperatures (Battery over 41C) at the beach, in direct sunlight, 90*F+ outside. Never sitting at home. I think the only reason this isn't a HUGE topic right now is because most people don't use multi-window functionality often. Its not that the phone feels incredibly hot. If you don't use pop-up windows, you may never notice it.
its just the phone, nothing I can do about it. Didn't have the issue doing the same exact stuff on the S20. I tested it quite extensively for 2 weeks. 40*C isn't even really that hot, but its 'too hot' for multi window.