One way of telling the juveniles of these two species apart might be tail pattern. Eaton
et al (Birds of the Indonesian Archipelago) shows juvenile Blyth's with 4-5 narrow, evenly-spaced bands on the undertail, and this is also explicitly stated in the text. This would be borne out by this rare image (there is another at no.9) of a juvenile in flight:
http://orientalbirdimages.org/search.php?Bird_ID=891&Bird_Image_ID=93168. Juvenile Wallaces's, on the other hand, is shown to have a broader terminal band and 1-2 bars only above that. The OP bird seems to exhibit the latter pattern.
I also found this remarkable webpage with some images of what are claimed to be juvenile Blyth's. The first bird (2010) is extremely pale, almost white. There are only three tail bars of even width, so I wonder if it's correctly identified. Then there are two images of another juvenile, taken in 2011, with a black crest, although the tail is not visible. The last bird, photographed in 2011, also looks pale and shows a dark crest - I don't know if that signifies anything. I assume birds on the nest were correctly identified, as the adults must have also been seen.
https://singaporeraptors.wordpress.com/blyths-hawk-eagle/
Lastly, the subadult on this page looks more like what I would expect a 2nd or 3rd cy bird to. The bird in flight on OBI is captioned subadult (2nd-3rd cy), but I find that hard to believe. Several images on OBI show birds on the nest in January or February, so could this bird have been fledged around then and could it be a 1st cy bird, about 6 months old?