Have you tried him on the kids toilet in a parents room - see if he will go on a toilet that is his size? Maybe if you can get him used to the toilet first. It sounds like it is that he thinks it is too big, maybe a step for him to stand on, have his feet on might help.

I know that DS happily goes on the little toilet and potty, but still has a reluctance to go when placed on a big toilet. This is only wees for us ATM, although we got the big breakthrough on poos last night with our first potty poo