Since we established that the relationship between the noun and its modifier is completely fluid, I guess this could also mean "My boy is not a son"?
Well not completely fluid; prepositive (modifier first) is still the normal order. But yes, it would be a possible reading, I think (it would sound somewhat marked, perhaps, like "my boy is no son!").
One has to be careful with agreement of the inflections; but here trēsy and taoba are both lunar nominative, and ñuha is ñuha in both positions.