A few weeks ago my cat showed up at home with a bald patch. It turned out that he was pulling his fur out and as it got worse I took him to the vet. She said he’d lost weight and was stressed. She advised feliway and omega 3. I’ve plugged in the feliway but need to wait for the omega 3 to come.
This morning as I was stroking him I realised I could feel his spine

The two things that have changed: his brother, who he was very bonded with, kept running off; we’d moved house last year and he kept going back to the old house for weeks at a time. We contacted several rehoming centres incl the one he came from for advice and all agreed he would never settle here - he’d been running off consistently for 8 months, no matter how many weeks I kept him in for and I tried all the tricks. We managed to rehome him with a lovely family as i didn’t feel it was responsible to keep having him run away - and he hated being kept in.
Thunder, the remaining cat, showed up with this first bald patch on the morning before his brother left, and he was always fine when his brother was missing so while I think he’s stressed through missing him, it could be the second factor - which is that my baby is now a toddler. The cat follows her, sits near her when we have a large house with lots of other options, and she strokes him gently - but she is loud and toddler-like (!) and I wonder if this is the cause of the stress?
Any tips or ideas? How can I bring his weight up? His fur doesn’t seem to be getting better although it’s more thin now rather than bald patches. He seems fine when he’s near us, purrs, plays, sits on us etc....but somethings obviously not right! Any help appreciated!