I 100% second what Annelies said, literally everything from the suggestions on lunging to the comment on nutrition!! Incredible advice you should absolutely follow!!
Remember every single minute that you're riding him, you are dictating his muscle structure. It can take years to correctly build up muscle in the right places and even longer to lose bad muscle. And do remember that some horses will simply never get bulky, their bodies just aren't made to be like that.
With all that said, if you really have worked incredibly hard to build the muscle naturally and you aren't seeing any results, I have heard really good things about SmartMuscle Mass, people have had a lot of success with it. https://www.smartpakequine.com/mobile/ps/11814
But remember, no shortcut will replace good training ;)
The best thing you can do for developing your horse's muscling is proper riding. A person can take all the protein or creatine they want, but if they aren't going to the gym, there won't be any change.
Make sure they are working from the hind end, and stretching down over the top line. You can also work them on a lunge line in side reins. My favourite way to set this up is to have a side rein on the outside bit, and run the lunge line through the surcingle/saddle to the bit on the inside of the circle.
Also make sure that their nutrition is optimal, or it will be harder for them to gain muscle over time. You can get your horse's hay tested for nutrient content, and you can look at their feed, and see if any changes or additions need to be made.
You should try "Muscle Up Powder" by AniMed. My friend used it for her horse and she told me it works it can just take time sometimes :)