Not that I've ever travelled with a baby but.....

1) When you order the taxi, you can request one with a baby seat. Not sure if they have ones for very small babies though.
2) When you book your plane tickets you can request a child seatbelt, which attached to yours. They aren't compulsory though. His sling might be ok. Best to check with the airline though...
3) I'd say you could hire a car with a car seat already fitted, but you'd want to make 100% sure that it's fitted properly.
4) If he's happy to sleep anywhere I wouldn't bother with the portacot. If he's fussy I'd say it'd be worth it.

HTH