10 POUNDS of Omolene? Seriously?! Wow...
I had my little one on Omolene 300 until she was a year, and de-wormed her every month too. But she only got a coffee can a day, personally, I can't imagine pumping any horse with that much junk food. Imagine you are feeding a child 10 lbs of ice cream a day. o_O On top of her Omolene, she got four flakes of Alfalfa a day over the winter, and about half a coffee can of beet pulp. She put on weight really well and filled out really nicely.
I would look at what you are de-worming with, and making sure that you aren't using the same thing everytime, so the worms aren't getting used to de-wormer. I know that being small means you can use the same tube a second time or even a third depending on his weight, but even opening a new tube the next month, and then going back to the first tube the next is better than using the first tube two or three months in a row.
For reference, Gracie at 7 months, when I first got her. [Take whatever you can or want to from the pictures. I would definitely suggest beet pulp, it's really helped her to fill out and even look like she has muscle even though I really haven't done any real work with her, lol.]
And at 9 or 10 months.
13 or 14 months.
And a month or two ago, at about 17 or 18 months.