1)Immigration first, he’ll need to queue in the ‘All Other Countries’ as he’s a foreign National, (The queues can be long, but if he's 'unaccompanied monor', they should get him through faster) then Baggage Collection followed by Customs on the way out.
2) Yes
3) His Uncle shall need to wait by the barriers in Arrivals for him to walk through.
4) He will be on his own once he gets to Security after depositing his bags at Check In.
5) I’m not sure what the procedure is on that one!