I suggest when you are able to ride to lunge him in a Pessoa system or side reins, this will help build up his gubd end. Also don't just do ring work if you have the ability to ride outside your ring and around the property or have a nearby trail definitely do that. Constant changes in the level ground is good for building up stifles.
It depends what kind of fitness, fitter as in stronger muscles for schooling or cardio fitness. You'll also struggle to get him fit if you can only ride twice a week. Increase your schooling time or lunge him in a pessoa for schooling fitness or do some interval training for cardio.
hay! for this i would suggest to do a lot of flat work. you can work on turn on honcho and side step as your troting! that how i got my horse fitter!
Hello! I will recomend you to do wind with he because it is the best exercise to musculate your horse pretty well. Wind with head no more tahn 20 minutes 5 each hand trot ann 5 each hand gallop!