Get 2 - 1" rings (or larger depending on what material you are using to make it). Then measure him from flank to flank to get the length of the breeching piece. Then you need at least one buckle strap that goes from the breeching over the hips (called the hip straps), attach thru the crupper you already have. I actually have two straps to buckle into the breeching. Supports it better.
Your guy looks significantly smaller than any of ours - can measure a harness breeching tomorrow. I simply don''t remember any of the lengths we have...
Here's some pictures of breeching that I made. This one is too long on this mare - I shortened it up on top by about 1" on each side and stitched it down but don't have any pics that show that. The breeching should be about where the black bands are holding the ends of the breeching straps (above the breeching).
You will also need hold back straps (the harness above doesn't have "holdbacks" - it is fashioned more as a work style harness not a pleasure harness). The hold back straps attach from the breeching to the cart at the "Footman's loop". That is what stops your cart when you stop your horse.
Here are the hold back straps hanging loose on this young mare.
This shows how the hold back strap is attached - but it is too tight here. Shouldn't be tight when the horse is standing still. There are also several driving books that show exactly how to "wrap" the hold back strap - it is done in a wrap that goes around the trace but allows it to move freely. Clear as mud, huh??
And I seem to have a lot of photos of how not to do things, LOL.