Few month old Stamped concrete patio crumbling in a few areas
It looks like maybe the concrete was allowed to cure too long before stamping and had started to harden excessively. You could grind the loose areas down, but they'll have a different appearance from the textured areas. 

If it was my patio, and I wasn't interested in pursuing satisfaction from the mason, I'd probably find an epoxy sealer that's designed for concrete and try to bandage it up as well as possible without altering the appearance too much. You may be able to tint the epoxy to match, and then fill the cracks in the crumbed areas to bond them together.