Either look into some dwarf cultivars of crepe myrtle or southern magnolia ('Little Gem' is one), or consider using a large shrub, pruned up like a tree (loropetalum is one). You know there are some shrubs that can get quite large.

And I'd suggest he plant it in the fall when it has a much better chance of survival against his style ("put it in the ground, water it a couple of times and forget about it" ).

One small tree that I like is the Vitex (Chaste tree). It has pretty, spiky blue blooms. It is pretty hardy but anything that is planted in the heat of summer MUST be watered regularly. (Vitex isn't especially slow-growing, but it isn't a large tree.)

Esh, I have forbid him to plant anything till fall!