I can skid all 4 tires on my '31 Town Sedan on dry pavement with the mechanical brakes. And this is with the larger V-8 wheels/tires on the car (more rubber on the road). There is nothing wrong with mechanical brakes when they are right.
The biggest problem with mechanical brakes is worn parts that the restorer is too cheap or too lazy to replace. When everything is right, like original, and adjusted properly, the mechanical brakes are really good. And all the parts you need are available.
One thing that Model A brakes need is occasional manual adjusting, to account for brake lining wear. It's a bit more often when the brake linings are new, until they wear in. Hydraulic brakes still have to do this, too.
I believe your son will have a greater sense of pride and accomplishment if he restores the brakes as they were originally. It is a different type of system, so it will require him to learn some new skills; nothing wrong with that. The pride of having restored it back to original will mean a whole lot later.
My opinion. YMMV.
