Although this is normal for horses his age at this time of year, the sudden growth in the length of his bones is putting a lot of strain over specific areas of his body. His bum is particularly high, which means he is sitting very down hill just now. This is putting a lot of extra strain over his lumbar region (the small of his back – just behind the saddle). Also the fronts of both his shoulders and both humerus – head of radius joints (at top of foreleg) are loaded up. So his trot is now heavy in front and his energy levels are shot to pieces.
If your horse is suffering the same issues, the most important piece of advice I can give you is: Take It Easy. Do less work – and especially less trotting, as it is the most concussive of all gaits. Back off general training too, until the back end matches up with the front, then ease your horse back into full workouts.
As a bit of light relief, check out this short video clip of Samson enjoying his treatment yesterday!