Firstly, the agency must be GST registered before it can be allowed to charge GST. Ask him for the GST registration no. and verify this with IRAS. If he is unable to provide you with this, report it to IRAS immediately for investigation.
Secondly, is it stated in the commission agreement that he is charging you GST? If it is not stated in the commission agreement that you signed then the commission shall be treated as to be inclusive of GST.
See paragraph 2.3
http://www.iras.gov.sg/irasHome/page03.aspx?id=2328
http://www.iras.gov.sg/irasHome/uploadedFi...inal_010707.pdf
Source: IRAS
If still in doubt, check with IRAS and if you still think your agent has tried to charge you GST in breach of IRAS regulations/guidelines, report him to IRAS for investigation.