At seven, sometimes soft, reassurance and openly noting his frustration can be helpful. He is at an age that can, sometimes, be reasoned with. If he settles enough, at that age, he has growing emphatic tendencies likely. Not that you want to share much, just explain you experience upset when he is upset. It is a difficult balancing act and such strategies can work, may work, be totally ignored, or interfered with by his parents or guardians. Best wishes and good luck.
The only other move I can think off is you will need use your coping skills and use radical acceptance knowing he won't, at least for now, change.
I can attest to having had success reasoning with kids that age when I worked as a TSS. Can't say it worked for my kids, but they all turned out as great adults; I probably mostly remember the tougher times with them as headstrong youngsters.
Wish you success.