It may be a generational thing more than any judgment. My husband sent his grandson cash for a graduation gift...heard nothing...and was told by his mom (DH's daughter), "He doesn't do thank you notes." Ummm. Okay, Mom, way to go on the raising.
We have decided that no acknowledgement, no gifts. It's about not wanting to reinforce bad manners...that will not serve them well in life.
Point is, maybe he's just rude or doesn't know any better.