I am in a unique position to be all three, Momma to two grown sons who are married and have families of their own, Momma to one son still at home and in high school, and Mom to not only 3 grown dogs but a litter of 5 puppies who I had to bottle and syringe feed from the 3rd day of their lives.
My grown sons will call me, tell me Happy Mother's Day and that they love me but usually spend the day with their own wives as they should. One lives out of state and the other out of town. My son that is still at home needs to give me a big hug, tell me Happy Mother's Day and how much he loves me!
Then he can go off with his friends and spend some time at the beach. He usually has a surprise for me as well. Each of them will do something to recognize the day and give me a little extra loving attention. They are great sons! I am proud of them and love them to pieces!
As far as all my canine kids -- they will spend the whole day with me! They won't take me to dinner, and won't get me a card, but I will undoubtedly get lots of puppy kisses! I will probably take my mother out to eat the day before Mother's Day as neither of us likes the long wait times at restaurants on Mother's Day. I already have her presents wrapped and ready. For me, it has never been a question, any time I have lived in the same town as my mom, I always spend some time with her on Mother's Day or the day before (and I am in my 50's!).
As much as I love my dogs, as much as we include them as part of the family, they are not anything close to being like my real kids or my own Mother. On Mother's Day, I want to be recognized for my 3 human sons not my pets. So, I would say a picnic in the park is fine, but not as a Mother's Day present -- just in recognition of wanting to spend time with each other. Let him still spend a little time with his mom too. I remember some years where I would take my mom and dad out to Brunch and then my boys would take me out to dinner later in the evening. Sometimes we all go to dinner together (my personal favorite -- especially when my older boys are in town too.) So, you can have your time and he can still spend some with his mother too.
Until you and your fiance do have real kids together, I think he should spend at least some time with his mother every Mother's Day and if it was me, even after you two have kids, I would still think it very appropriate for him to do a little something for his Mother on Mother's Day. There should always be a way to BOTH "cleave unto another" and still honor your mother. IMHO. Try not to make it a competition.