Been there. Eventually we had to stop letting him leave though because he brought fleas in. Twice.
Actually it might’ve been three times
The cheap flee collars do not work, and if the cat was even semi-feral good luck getting any collar on it. Tropical is usually best, but stuff that works is also not cheap.
I often find going to the tropics solves most of my problems!
In all seriousness, I totally agree that topical is the way to go.
What does work are anti flea drops you put on their neck, where they can’t reach. Lasts a month or so and kills fleas and ticks. My mum’s cats are outside a lot and they never have fleas or ticks, when my mum doesn’t forget putting the frontline drops on.