Since I'm kind of creating these ideas on the fly, I guess I would definitely say whatever seems to resonate to you as the best idea would work. For me, it seems to make the most sense that you would make a 'his' and 'her' potion -- one that is symbolic to each of you as individuals... to boost each's sexuality/fertility (since I'm assuming this will be consumed/used pre-copulation), but now thinking about it, perhaps you could each choose one ingredient to add to the other's potion

And of course, this is the recipe for the individual potions... I think after creating them, you would do a ritual together in which you both do the same things before/during/after consumin the potion itself.
That's just what seems to call to me... you can even make copulation part of the ritual, but I don't really think that's necessary, or if it is, you don't have to view the copluation or sex magic as the attempt at conception, if that makes sense.
If you want I can PM some things to look for to get hints toward male fertility. Since it is specific to inspecting one's body and fluids, it can be kind of TMI/gross to a lot of people, so I don't want to just post it publicly, especially if you don't want to know it XZD It's not all-telling or anything... just some things you can look out for (or rather, he can look out for. It's not much you can notice yourself.)
Some easy things you can do to increase male fertility (every increase helps for the male or female!!) include meditation to reduce stress (regularly, not just before sex), eaitng a vegan or vegetarian diet (or even just a balanced diet -- at least half of everything you injest should be veggies, and avoid red meats), having lots of antioxidants (white tea, green tea, acai berry, are all popular sources of antioxidants), regular exercise (especially if he is obese or out of shape), and regular and safe sexual activity. Also, wearing loose clothing around waist and groin, and not sitting down very much (or sitting on very soft things -- use a cusion if he does a lot of work sitting on hard or firmly-cushioned chairs!)
Our culutre lends itself to a lot of habits and lifestyles that actually work against healthy, fertile sperm, from jobs that require a lot of uncomfortable sitting, to prioritizing meats and grains over fruits and vegetables.
(Another big area of interest to me after tea is human sexuality, lol. And being male myself, I tend to know a lot about male sexuality and sexual health...)