Due to the fact that I show heavily with my stock, haybelly was my biggest frustration, especially in the weanling classes! When I first started showing, I noticed mine was the only one to have haybelly, while everyone elses were dang near perfect! Thus, my quest to study and research why began!
Looking at your weanling, and any other horse as a matter, grains work on the "top line" of the build, while hay/grass works on the "bottom line" of the horses build.
When you feed a grass mix, this is what gives the haybelly apearance. It is not anything to worry about when it comes to their health. It is asthetic only.
Now, speaking appearance only, this is what has worked for me and many others in the show world of weanlings. Depending on your budget will determine how far you want to go with "fitting" baby. Weanlings especially up to the 6 month age, will for the most part have this haybelly appearance as their bodies are coming together. How much you want to reduce it is up to you.
One thing I had to do starting out, was reducing the amount of hay/grass I was giving. I was even on a free feed plan which I had to cut out. Switching my filler product was the next thing. I moved completely away from any grass/hay mix. The grass was countering everything I was working against. I feed alf-alfa and generally a 2nd cutting. When it comes to show season, I trim it down to 1 flake a day. During the winter when they need to keep warm, I will go 2-3 a day.
I use a complete feed grain. Now there are tons of great brands out there that have lots of needed (and then some) nutriants. Which is going to be based on where you live with availability. Here is where I get a lot of shocked looks, but there is so much into whats going on as to why I feed the amount I do. I feed a regulated amount of 6lbs a day. I have also added alf-alfa pellets to the daily diet to help suppliment the amount of protiens and other nutriants I want to make sure they get.
I hope this helps some, and if this is a big enough issue with you, that maybe it can start you in the right direction. There are many other things a person can do to improve the appearance, but thats a whole other post! LOL! Remember that if your weanling gets a good amount of exercise, you will need to account for the amount of calories burned daily and adjust feeding accordingly.