This word seems unrelated to the same word in other close languages. In French a boy is un garcon, in German ein junge, but English remains aloof with a boy. This is not the only word of relationships that stands apart from affiliated languages. The closest I can find is the Swedish Pojke which would seem to imply that it is a word of the Northern Germanic languages including Saxon, unless it is taken from the Finnish Poike.
My suggestion is that the young men, the teenagers, were sent to look after the cows. Younger boys looked after the sheep – as in the child’s rhyme ‘Little Boy Blue come blow up your horn, the cows in the meadows the sheep’s in the corn’.
While cattle are ubiquitous nowadays, being directly responsible, at least the farming of them is such, for the loss of Amazonian forest to produce cheap burgers for questionable food chains, we may trace their domestication to the early days in Classical Greece – not to say this is the origin of such but that the place names carry their advent. Boeotia is clearly related to the land of cattle.
Records exist from Egypt in which the cowboys are seen as rebel-rousers, rowdy and destructive. In the same way we use the word today of someone who is slapdash, or worse, about their work. The same reputation they carried into mid-west America. Cowboys were not reliable and likely to cause trouble.
For myself I believe this is the result of working with cattle themselves. They are large beasts and prone to high spirits. They kick up their heels playfully and, wagging their heads, run at you.
When we look further in Europe we find Bohemia, as the land of the cattle again. Wherever we find Bo- we may look for a relationship to cattle.
For me the derivation of the English word is simply this – the one’s associated with cows. The Bo-ii, the boys and boyo’s of south Wales, who easily become yobo’s by simple repetition.