It isn’t really possible to compensate for too narrow saddle but sticking more padding under it. It sounds like the saddle may have too much angle to the panels. Like if you sight down the channel from the back, do the panels angle down, or is there a nice flattish surface? Friesians tend to have well-sprung ribs and need the panels flatter. Since he is going to grow and change a lot, you might try out a Thorowgood saddle - they are synthetic and hence much cheaper, and have interchangeable gullet bars. The cob model is made for broader horses: https://www.smartpakequine.com/pt/thorowgood-t4-cob-dressage-saddle-16679
If you start a new thread with photos, you can probably get some good insight here.
Incidentally, my 17H wide load warmblood is in a Patrick saddle built on their “cob” tree, so don’t let the description sway you. Black Country and Lovatt & Ricketts are also good choices for a wider saddle and can often be found used for lower prices.
And lol @sigrid.sijtsma as if warmbloods aren’t having OCD chips removed left and right. I know an Arabian with a stifle OCD. It is in all breeds and the WB breeders from what I’ve heard are routinely getting OCD surgery for about 60% of the two year olds and definitely not selecting those horses out. When the PPE on my mare turned up a coffin joint OCD, I went ahead and bought her and did the surgery because her hocks and stifles were clean, and the vets said the OCD she had was very operable.