What can I do to reassure my bf that he is able to ejaculate in my vagina and actually enjoy the feeling?

If he enjoys receiving oral sex I doubt very much he wouldn't like the feeling of vaginal sex. I don't understand why you have to do this in the park though. Couldn't you go to his house instead? I think that if he wasn't out in the open he would feel much more secure and confident of his abilities in making love to you. If you are still having trouble I suggest you make the first move because I'm sure once he is in, he will enjoy it thoroughly. Good luck with your genius un-conceived child.