by stocked up, do you mean his back legs will swell or seem to be filled a bit?
This happened to my gelding all-the-time from when he was 2 - 4 years old :) It sounds to me like an old-fashioned case of getting sore after a work out, then standing still.
Think of it this way.. when you go for a jog, you may feel a wee bit sore when you come home, but usually, its the next day when you feel the pain in your muscles. When horses get done working out, the blood is pumping and everything is flowing well. Then, when they stand, it causes things to settle out again, which may make the cannon bone area on the legs look sorta puffed out.
My best advice, and simple cure if this be the case.. is walk him out (by hand or in saddle, either or!) to get the swelling to go down. We were out every single day to see our gelding, and we would hand walk him, or bareback walk him for 15 minutes or so to promote the swelling to disperse out.
I think your guy is totally fine :) If you do get pretty worried though, it never hurts to call the vet and ask him for an over the phone opinion (followed by many thank you's!)
The advice and input I have given was from my personal horsie experience, im no vet, but from whatcha described, that's sort of what I think is going on =) I just wanted to mention that ^^
Our gelding cast himself once really bad too, and bruised his back legs, but it never made the swelling worse =) He got stuck BAD too x_x