For an upright bike, I wouldn't worry about visibility and different tire sizes.
A recumbent trike as mentioned above has much better stability, but may or may not compromise visibility. You can always add a flag or two if you think it is necessary.
What kind of riding do you have in mind? Distances? Paved vs gravel?
Many of the trikes drive on one wheel, which is a pain for dirt or gravel, and may feel like it is pulling to one side.
Many of the trikes are 1-speed which limits them to shorter rides around the neighborhood, and perhaps also limiting hills.
What about storage and transportation? When I was looking at trikes earlier DiBlasi was the best folding upright trike, although they are expensive, and HEAVY. It appears as if several recumbent trikes are now experimenting with folding.
Anyway, much of the innovation in the trikes is in the tadpole recumbent trikes, and can be very high end bikes. Expect to pay well in excess of $1000, but if you want something that he can ride and keep up with others... that would be the way to go.
As mentioned, above, the upright trikes like the Worksman are also designed for some cargo capacity which can be nice for trips to the store, although many bikes can be adapted to carry a little cargo, or perhaps pull a trailer.