It does sound as though he needed one-to-one attention and fingers crossed he will bounce back.
Feeding him extra food like dry porridge oats (just a half teaspoonful) over his normal mix and buy a bag of Canary mix from any pet shop as this will contain hemp seeds, linseed as well as mixed millets. Although I`m sure he already gets millet if you feed him the Burgess dwarf mix.
Will you be keeping him on his own now? I feel it`s probably best now in case living with Cloud, he may have been being mentally bullied rather than physically. It`s hard to say why he would go like this while Cloud is fine. And illness in one of a pair can lead to the fitter hamster starting to push his weight around for dominance if they sense their weaker counterpart is ill.
Let us know how Smokey is doing.