I don't have too much additional information, but I found a little more about Jon Shannon:
-- a 1962 San Fernando CA directory records: "Shannon, Jon (Joyce) Designer, 13249 Kismet, EM 7-7440" (NOTE: I don't know if that is the same man--and his wife)
-- an article about a 1968 Los Angeles fashion show noted: "Jon Shannon wearing a shirt he calls "Early American Bedspread" and Ray Ross in a silk shirt all big ruffles down the front (got it at a shop called "Grandpa Takes a Trip").
-- a 1968 Los Angeles article about boutiques reported: "Grandpa Takes a Trip was started by Raymond Ross and Jon Shannon as a tiny shirt shop for men. It has burgeoned into a 'his and hers' boutique on Melrose."
-- a 1969 article noted: "Grandpa Takes a Trip is a boutique dealing in apparel and antiques..."
-- I found the 1970 references to movies for which Shannon was the costume designer, including one called Blood of Dracula's Castle.
-- an April 1970 article about movies noted: "Ann-Margret's most feminine-looking (are you listening?) gown was designed by Jon Shannon, a young California designer who will create her elegant wardrobe for the movie, in which she plays a high fashion writer and coordinator..."
-- a 1970 Los Angeles article on style reported: "The new names to watch in California include...a boutique line from Jon Shannon..."
-- a 1970 Los Angeles article about owner-designer labels reported: "Jon Shannon (formerly Grandpa Takes a Trip) 8590 Melrose Avenue. Beautiful After-5 clothes and customized men's shirts in an organically decorated boutique. Owner Jon Shannon designs everything under his own label. Last season patchwork was the big look. This spring it's a Shannon interpretation which he calls "Edwardian 30's" -- long, soft and feminine. Fabrics are very important...Other goodies include caftans and burnooses, a newsy line of men's shirts, and a selection of antique jewelry and Liberty scarves. Most clothes are made to order from samples. Long dresses are priced from $95 to $300, pant outfits from $95 to $165, long skirts from $45 to $125, and men's shirts from $20 to $35 (or is that $85?)."
-- a 1971 Los Angeles article on fashion shopping reported on the Jan Roth shop in Beverly Hills: "Jon Shannon, who designs exclusively for us, is the only label we feature." says Roth..."
I found nothing about Shannon after that, so I have no idea what happened to him after the early 1970's.