Okay, let's do the math
1000lbs horse + 180lbs current trainer + 45 lbs tack = 1225lbs : 8(circumference of midcannin he should have due to his height) = 153.125 : 2 = 76.56
Okay, but not optimal.
Still okay but pushing it a bit
1000+260+45:8:2 = 81.56
So, without exactly knowing all measurements, like tack and cannonbone, you would have to find ways to reduce the overall weight, lighter tack, for example(cordura saddle instead of leather, no heavy pad, just a blanket, for example.
Plus it would be interesting how he is built, like length of back, loin area, narrow or wide in chest to start with.
I am in the same boat, having a small guy with 14.2, but very short and close coupled.
That's why im naturally interested