This is why it is always fatal to be professional. It boggles the mind that you did not flirt with him before, but I do believe you. How, though, will he ever believe that you feel something now when you gave no hints before?
flirt mildly with every colleague, man or woman, young or old, expressly so that they will never be surprised when we have that 'I have feelings for you' schtick at the end of term drinks. That philosophy makes that old maxim about never working with children or animals even more important, of course.
More seriously, I agree with those saying you cannot be overly subtle. I stand with Arwen on this one:
'If you want him - come and claim him'.