Fantasies are strange things in that there are some that prove attractive because they will never become reality, and others that one is desperate to try out. It is a feature of some fantasies that they have an element of privacy, and the involvement of a partner, especially one who is less than keen, can burst the bubble, so to speak.
I understand the OP's desire to inject some passion into her sex life; there is something really wonderful in having a long-term partner who has the hots for you. Involving herself in his food fantasy may be a way to do that, but I do detect that although the OP is eager to raise the levels of passion, she is less than keen on the food fetish idea.
Her language tells us quite a lot. I am no psychologist, but the phrase he 'erm enjoys himself occasionally' is a euphemism that reveals a somewhat puritanical attitude to his masturbation that is probably also reflected in her approach to her partner's fetish. She tells us that they have watched some food porn together and her partner in all likelihood has picked up on her subliminal mindset. Why would he seek to involve someone in his fetish if he knew their heart was not in it? The OP tells us that she doesn't find the prospect of being covered in custard and cream anything but sexy to her. She tells us that she'd be prepared to try it if it was important to him, and that sort of approach is hardly likely to result in his having a raging erection. If she is prepared to engage in his fantasy he has to believe that her heart is really into it, even if she isn't, so a bit of acting is probably required. His ED may hinge on what he thinks is her reaction to his fantasy.
When watching food porn with him her sentiments about being covered in custard in all probability come across. I think the way to approach this may be to adopt the view that while being covered in custard isn't that appealing, the prospect of her partner being overwhelmed with desire for her should be the goal, and that certainly ought to be appealing. The end justifies the means, if you get my drift.
We are also told that he uses Viagra for ED/low libido. As far as I know Viagra has no effect on low libido; its sole purpose is to enhance blood flow to the penis. I am a user of Viagra and sexual desire results in a dose; a dose doesn't result in sexual desire. The time that Viagra needs to take effect and its necessity to be taken on an empty stomach mean that planning is required and spontaneity goes out of the window, but it can be done. His food fantasy also needs careful planning if the house isn't to resemble a chimps' tea party, so perhaps the two factors can be planned for together.
It is also important to not confuse quality and quantity. Low libido may result in infrequent sex, but it could well be mind-blowing when it actually happens. A high sex drive could result in lots of sex, without it being of great quality. I suspect that she wants sex more frequently, but with much more passion.
The OP indicates that she does not want the relationship to die. She feels that realising his fantasy may be the key to having a more frequent, more passionate, sex life. It may indeed be the key to getting where she wants to be; it must also be borne in mind that it might not achieve what she wants it to. It is notoriously tough to align someone's varying sex drive with yours. Having said that, a job worth doing is surely worth doing well: she needs him to believe that she is as consumed by his fantasy as he is (even though that is definitely not the case). Approaching the project with a 'meh, I'll give it a go, but I am not that keen' attitude is absolutely not going to work.
Best wishes.