You could try being really submissive during sex and making sure he is aware of how much you like it. I am not talking about calling him Master and so on but maybe letting him know that this time he is totally in charge and you will do anything he wants. You could even have some toys ready for him to use if he chooses. May work better after you have shown him some stories or even discussed it with him openly. Also, don't make it a pressure thing and don't be disappointed if he does not perform to the standard you want. Just have fun and build things from there.

I do have to ask one question: How would you feel if he kept bothering you in order to have sex outside with him? I am hoping with this question I may be making you consider his side of the situation. How willing are you to break what you suggest may be a limit in order to make him happy? That may be exactly the same willingness he has to do the same for you.

Perhaps consider a deal with him. One day a week (at least, more if you can manage it but work etc may interfere) when you make it a rule that you take it in turns to fulfill each other's fantasies. Start by sharing them first and giving each other time to plan (and you can keep your plans for him secret and he can keep his for you also secret so long as you both follow each other's general wants) then you can do whatever you like. On 'his' day you could prepare a night of sex outside (for example, you may decide to follow a less scary fantasy for you on the first time at least) and he could do something BDSM for you. The keeping it secret bit is not essential but adds to the fun and anticipation.