I'm kind of surprised that this isn't the popular answer, but if the kid wants to play trumpet, let him play trumpet.
Various reason for this, including that he might actually practice the instrument and get better on it as it's something he wants to do; he might detest the instrument you choose for him and it could put him off completely; he'll still be learning to read music as he plays and you can cover any other theory you feel he needs to learn on the side; and you're a trumpet player yourself so will be able to help him, make sure he's playing in tune etc.
Finally, why deny the kid the chance to have a go on something that he might end up loving for the rest of his life? That, to me anyway, makes no sense at all.
(Just as a side note, starting aged 7 is extremely common in the UK and is pretty much the standard starting age for most young people's brass bands.)