There is no oficial circuler...in that case you have to make him read and understand law which will be a real pain........but let's try......
Following are the provisions/rules which suggest that only service tax can be leviable and not gst-
1. Rule 3 of Point of Taxation Rules' 2011 of service tax says point of tax is the date of payment or date of invoice, whichever is earlier- in this case date of payment is in may, hence point of tax is in may.
2. Instead of point of tax gst use the term time of supply. Sec 13 of cgst act deals with time of supply. Section 13 opens with "the liability to pay tax on services shall arise at the time of supply determined in accordance with the provisons of this section". In summary section says if advance has been paid than the time of supply is the date of advance and the date of invoice, whichever is earlier. As again date of payment of advance in may, the time of supply is also may.
Now both acts says that tax is to be paid in May. And GST came into force from July. Hence government has no jusrisdiction to ask gst on this payment.
Instead of giving him proof you should ask him to give you proof that where is it written in gst law that gst need to be paid in this transaction.