Wow I have never heard of someone being so proactive in avoiding interaction with a guy.. Have you held a conversation to create a common interest? or some common ground? if so while you are still in the shy stages can you use them to begin a conversation? or can you talk about something in the shop to get a conversation started. and if you dont want to be so foward to asking him straight up if he has a girlf maybe use some initiative ie: I work in an automotive industry so women come in and will buy something and if I am interested to find if they have a partner I will ask "if your boyfriend is going to fit this part they might need"...or "is your boyfriend/partner fitting this? because we can fit it if you like " etc etc people will almost always correct an incorrect statement about their relationship status, especially women. (in my experience)
I dont think getting your mum to pass letters is the right way tho, because it would make it a bit odd to know how to reply.. if he is as shy as you say he might be finding hard to know what to say?
Guys love the compliement of a girl being keen on them, so even if he does have a girlfriend nothing ventured nothing gained?
goodluck
