I think maybe distractions? Whether you live in an apartment or house, if you could set up a bird feeder outside to attract birds for him to watch from inside, that could help. There are also some robotic cat toys that might help: https://www.amazon.com/PetDroid-Interactive-Attached-Feathers-Available/dp/B07WHSGF4T/ref=sr_1_5?keywords=robotic+cat+toy&qid=1667684243&sr=8-5 My cats got bored of it eventually but I think it's helpful in the short term for the time gap. You could even turn it on right before you leave. (My mom's non-sib cat was scared of it though so just make sure your cats like like like my sibs do.) I got a second cat too for my first's separation anxiety and it fixed it!