What a sweetie! Evidently purring IS something you learn as a kitten...over the years I have had two cats -- both of whom were strays--who didn't purr. One learned all at once at about 5 years of age and made up for lost time ever after. My current kitty--Bob--tries very hard to purr, but just can't seem to get the knack! I think it must be like whistling--once you get the idea you never forget...
So--don't give up! your guy may still start to purr--any time.

PAW