PECs

Hello, I hope I'm posting in the right place! 

I am currently working 1:1 with a little boy with autism who is 3.5 years old in a mainstream nursery. I've been working with him for a year now And went through the whole diagnosis process with him just before Christmas.

We have already been through the object referencing process and recently moved onto boardmaker symbols. As he responded so well to the symbols and tries hard to communicate it was suggested that we start using the PECs system. He has just started to talk and says 'hi', 'bye bye' when someone is leaving or he wants something to finish, 'uh oh' when he drops something, 'let's go' and then all the number and letter names 

So last week the SENIT inclusion workers came out and taught us the proper PECs process and tried it with him. We used sweets and he got very angry and distressed that he couldn't have all the sweets so we moved onto a ballon. Although he enjoyed the balloon he refused to give the card and threw it across the room (yes it was a struggle to keep a straight face!) 

Today I thought we'd have a fresh start and try again, he's not as familiar with the inclusion workers so I thought they may have put him off slightly but still no luck. I tried today with balloona and cars. At one point he picked the card up (I started to get excited!), very calmly put it back into the folder, said 'bye bye' and put it back on the shelf where I'd got it from!!! I didn't know whether to laugh or cry! 

So, long story short, has anybody else's child showed this much reluctance with PECs? Anybody any tips? It's very early days but I don't want to put him off. He's so bright and i know he is very able, I really do think he's just being stubborn. I think he thinks his communication system of pulling me and signalling at what he wants works so why change it?! 

Sorry for the long post, thanks in advance for any help!