Visitor Visa - B2

My boyfriend is currently studying in Germany, but is from Iran. He applied for a Visitor (B-2) visa to come visit me in December, but was denied due to 214(b). He has no professional job or family in Germany, therefore no strong evidence to support his claim that he will return to Germany after visiting my family. My father wrote a letter of invitation which he showed the embassy at his appointment, but he was still denied. I am prepared to fill out the forms which take responsibility for him while he is here in the USA, but I do not know if that will help. I can send many letters to the US State Department and my Senators, but my boyfriend's situation remains the same in Germany, with little evidence to prove he will return after his visit. ANY ideas or tips anyone could offer would be very appreciated. Thank you!