Is there a bus from guangzhou south station direct to kowloon?

No, the direct cross-border coach between the two sites are not available. I suggest you take local subway line 2 to go to Yuexiu Park from the South Station. Then leave from exit C and walk about 350yd to Orient Hotel, in front of which you can take a cross-border bus to HK. It will have some stops in HK, so ask the driver to drop you off at the Kowloon Area.