Actually, it wasn't that he didn't feel comfortable wearing his mother's tartan. He told me that he really wanted to wear her tartan, but was told once by the proprietor of another kilt shop that he was not allowed to wear it. He had to wear his father's tartan.
Matt, I have been told the same thing. Over the years I can think of a couple of local stores where the proprietors would have been quite adamant about this (that only Dad's tartan could be worn). At the time I took that as gospel! I'm glad he ended up with what he liked and felt good about it.
I guess I tend not to think of tartans as my mother's or father's, but as my own (but not in the exclusive sense). For example, if I had been born to a single mother named Oliphant, and that was my surname too, then that would be the clan tartan I would want to wear.