Congrats on your engagement, and on becoming a step-mom. You're so lucky to enter this little guy's life when he's so young - years of bonding ahead of you. :-) And sounds like he's lucky to have you in his life, too - already attentive to his needs.
The other moms are spot-on - developmentally, he's at a "I want mom" age. He may also be picking up on what's going on around him - wedding plans, maybe changes in visitation or in the prime relationships in his world. You're probably wanting to be the best step-mom you can be, but congrats for approaching that from his point of view, leaving him as the focus. Here are some concrete ideas that may or may not help.
As you've discovered, always helps them to know when they're going to see mom next At that age, not a great sense of how long a day or an hour is.......maybe a calendar (even of an advent-ish sort....remove an object from a shelf ea day, when the last object is gone, it's time to see mom, or something like that?) Or, when he's older, incorporate it into learning to tell time.
Making something together with him for his mom - a picture, a cake, an e-card, etc? So he knows (and she knows) not to feel threatened - that you're not trying to "replace" his mom. Also helps in recognizing positives in your fiance's ex that might, depending on your situation, get buried in animosities.
Is there something "special" that he could keep only at your house? Maybe a certain stuffed animal or blanket that he could hold when he's really missing his mom? Or an activity with you that he looks forward to? A trip to a certain park? Pancakes with sprinkles? (To which, inevitably, he'll reply, "That's not how my mommy makes pancakes." lol.) Shrug.
It will get easier as you all adapt to new routines and arrangements, though with some kids, any age, on an every-other-weekend schedule, you can expect a bit of bumpy "transition time" ea visit. He won't always be so focused on "when do I see mom?"
Enjoy your little guy and your new life together, and welcome to the wild, wacky world of blended families. Sounds like you're off to a great start!