While one hopes that CL has a lot of upside development still, there is NO guaranty to that. Quincy Wilson (another prodigy) ran 44.2 two years ago and 44.1 last year. One hopes he will make a jump this year, but I also wouldn't surprised if he didn't. The records lists (more on the girls side) are filled with athletes who peaked (never ran faster, even while competing for a few more years) at U18. Perhaps they just had poor coaching that prevented their continued development, who knows.
Now CL is already at a very high level (World champion), better than QW relatively, although he ran a 45.37 recently (almost losing to another HSer) where the World meet was won over half a second faster. Other than DL meets, there are not any Championship meets for CL to get into this summer (the world U20 meet would not be a real test, even if he does it), so we will have to wait until next year. If he gets consistent at that 1.42 level (1.44 indoors seemed almost comfortable), then ones hopes that he can make the jump to sub-1.41. Hopefully his coach will work on both his speed development and endurance as well.
I can see a relationship of 400m going to 1k as follows: 400 - 45.0 (+3sec) 600 - 1.12 (48) +2.5sec 800 - 1.41 (50.5sec) +2.5sec 1000 - 2.12.5 (53sec) that is 1.46 800m plus another 200m.
Perhaps in his 20's he can see what his 1500/Mile brings. But I wouldn't seek that out unless he starts stagnating at 800m. As to sub-1.40, much like Keely Hodgkinson, it may come down to how much speed does he need to get out in 49 and endurance to also come back in 50 (=1.39).