Hi there, welcome
Usually when a bun has fleas, you can see them if you comb the bunny. The fleas will jump up as you comb. They will also probably act a bit itchy! Even if he has them, it’s not super urgent (unless you saw him just swarming with them, but I doubt you would be asking here if that was the case!), so I think letting him settle in is fine.
If you choose to treat for fleas as a precaution (which isn’t a bad idea necessarily), do NOT use Frontline. The active ingredient fipronil is toxic to rabbits!
Advantage and Revolution are safe to use, and I believe the weight dosage is the same as for cats. I do find with bunnies you have to put it more on the base of their head on their neck, vs. on their shoulder blades, so they don’t groom it off.
Since he is a new pet and was kept out doors, might be a good idea to get a check up at the vet. Then you vet can let you know if he needs flea treatment and you can get some good baseline info for him. 
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.