I agree. No worries about BRU, but I too would be very concerned about 8.5 hrs for a 14yr old alone at BOM. Perhaps someone can collect him from international arrivals from BOM and keep him company till the departure of the CCU flight, and be met at CCU arrivals.

With all the various con artists offering different transport options, 1he transit between BOM international and domestic landside can be intimidating for adult tourists, let alone a 14 yr old.