It probably works better to work a few fences into a few of your flatwork days. Like, maybe, 1 or 2 jumps that you can move around each session one vertical, one oxer, maybe jump them 2 or 3 times each but built into your flatwork instead of just doing flat some days and jumping courses on others. Set the fences worked into your flatwork low. When you do your serious jump school days, set a little higher with some at your competition height but don’t overdo it, if he’s good, quit.
I’d set up some kind of low natural obstacles in that paddock, like logs or a few hay bales. Just set randomly so when you hack out you can pop over them from any direction, it’s fun, keeps horse from getting bored and anticipating repetive courses. You have to keep it fun and interesting or you’ll both get bored.
Sooo…
Monday, Hack out or light arena ride, no jumps.
Tuesday, Off.
Wednesday, Lesson or Jump exercises like a grid, circle, etc.
Thursday, Flatwork, hack out, a few jumps
Friday, Off.
Saturday, Serious flatwork, no jumps.
Sunday, Flatwork and jumping exercise.
Note, a few jumps means a few. Hack out means out of the ring. If you don’t have any tracks to follow, use the paddock and hilly area.
Anyway, I did this with my 3’ Hunter in its mid teens. Worked quite well for condition and sharpness for both of us.