Reply to a professor who said no open position BUT admired the work

I have asked a professor in germany for PhD position in his lab and I sent a mail with previous work experience, future research plan and why I want to join his group within 8-10 lines also attached a simple CV. He replied me starting with 'You are a strong candidate' and he admired work and plan also, but he said presently there are no open positions.

But I want to work under his supervision. I have at most 6 months to complete my masters. How can I give a reply mentioning I have time and in that mean time if he found any open positions I can join his lab.