16 year old son dating (detailed)?

Our son has been talking/texting/facebooking with a girl at church. They've been talking for about a year, but she wasn’t allowed to date until she is 16. Or at least that is what her mother told my wife. Even then it has to be chaperoned. They each turned 16 in June. He asked her if she liked movies, and she told him she loved movies. So my son stopped her dad after church and asked if he could take her to a movie with us. His dad told him no, that she isn't allowed to date until she is 20. Of course my son was heartbroken over it. He said he told her dad that he understood and he'd wait.

What is awkward is my son is mildly mental retards. He had a 107.9 temp when he was five and we rushed him to the ER. You wouldn't realize it talking to him. He is more social adept then I am, as I'd never had the nerve to talk to a girl’s dad. But for school he cannot get above a third grade reading or math level. He did help the church last summer put on an addition. He just needs someone patient to explain to him. People at church know and understand him, as does this girl. He is very gentle and kind hearted. He said she liked him because he isn’t pushy like the other boys. I know I tried to teach him not to be ashamed of who he is, because it is what you do that really matters.

My concern is I believe this girl’s dad doesn't want my son dating his daughter because, in the long term, if they stayed together, my son could never give her a good life. He won't be able to go to college, or graduate high school for that matter. And if that is his concern, I can sort of see it.

My question is how to handle it with my son. Usually I am straight forward and honest and try to help him understand the situation and people. But affairs of the heart are different. The dad doesn't want his daughter dating my son, and adding four years to the dating restriction is a subtle way of doing that. My son took it literally, and I would hate to see him waiting for nothing. And I hate telling him why.