Haha well I'm not offering you predictions accurate 2 hundredths of a second (lol) or saying his 1:48 800m was actually worth 1:46 with drafting to the bell and speed work!
I'm simply saying that his 800m is an outlier in terms of quality. For a 3:59 miler to run a 48.5 FAT, not a dodgy closer-to-395m-with-a-rolling-start-hand-timed relay split, suggests they can run a very good, much better than 1:48.7 800m.
If a runner achieved a sub 10 100m and a 44.0 400m, would you not expect them to be able to sub 20 for the 200m? The marks above and below 200m suggests they should do. You could also pull the conclusion that perhaps 200m would be their best distance.
This is much the same.
It is possible that he is just weak at the 800m, has trouble pacing himself correctly/the rhythm of the event/cannot deal with the specific energy system requirements, but after watching his solo 1:48 I think that is unlikely.