HWT is an evidence-based program. It can certainly help for kids with a variety of handwriting issues, but certainly not all.

First you need to know why his handwriting is horrid. Is it developmental, a lack of practice or instruction, or are there fine motor deficits? If the teacher is recommending such a program, then the teacher should be requesting the school OT do an evaluation. Is she trying to end-run the potential need for services?