Personally, at 5 months, I'd leave him be! And I'm a fan of thumbs over dummies tbh, dummies can get lost, at 3am I didn't need to go in and re-find ds's thumb for him! :P

Whatever settles them is fine I reckon. I'd say that many children grow out of thumb sucking as they grow older, and those that don't can be persuaded by other means.