Hi Alexandra, I'm sorry to hear about this. My name is Linda, and I'm a small animal vet with 10 years of clinical experience.
A once off vomit is not usually a concern and can be due to e.g. dietary indiscretion ( a cat eating something they should not), eating too quickly or an abrupt diet change.
However, if the vomiting is ongoing, we'd take it seriously; especially at his age.
At 9, we start to wonder about things like kidney disease, an overactie thyroid or dental disease (which can lead to food bolting and subsequent regurgitation).
A vet check is sensible id the vomiting is continuing. They'll examine him and may discuss some tests such as a blood test and abdominal scan, to get a clearer idea of why this is happening.
I agree a probiotic won't do any harm. I'd feed a bland and digestible diet (like chicken and rice) and use a slow feeder bowl if he eats quickly.
If prone to fur balls, consider a fur ball paste and be sure to brush him daily.