I have a 3yo that has been on stall rest for 3 months due to severing his extensor tendon. I’ve started hand walking with light work (trotting on a lunge line), and he is losing his s*%!. This gelding has always been calm cool and collected, always willing with a very level head. He only had around 20 rides before he was injured but he was a dream under saddle. Someone please tell me the hand walking will get better?! He will be calmly walking on a lose lead then randomly blow - rear, buck, jump, bolt and then stop and continue walking like nothing happened. He also does this while trotting on the lunge line. I’m at a loss of what to do since he’s never been like this, and it’s making me a little worried to climb on his back again for the first time. :/

Sounds like he just wants to play. Might have a little cabin fever. I know I do!

Talk with you vet and tell them the problem they can give you something to help with him.  It will keep him from hurting himself and you too.

Poor guy has been all cooped up and now hes starting to feel better and get to work some! I'd be excited too. For things like this I like to use a rope halter so when he does get to bucking or acting a fool you can pop that nose and get his attention a little better than you can with a flat nylon one. 

A colt on stall rest is like locking a 5 year old child in a closet with a gallon of Mt Dew. They're going to lose their sh*t. When it happened to my 2 year old a few years ago he was put on Calmex and it knocked the edge off just enough that he didn't get stupid doing the rehab. 

What is his diet? 

Free choice local hay, 1.5 lbs soaked alfalfa pellets & 1.5 soaked beet pulp in the AM with Platinum Performance and Purina Outlast, .5 lb of Renew Gold & 1 lb of Haystack Special Blend, Platinum Performance and Purina Outlast in the PM. 

Oh boy -- I'd stop the alfalfa and the Platinum and put him on a senior feed with free choice grass hay.....and feed him Calmx or something.... stop fueling the dragon :)

I would just feed a pelleted feed as in the Renew Gold feed and stop all supplements for now and feed vit E and all the hay he/she wants for now. 

My vet gave reserpine to get us thru about 4 weeks.

Try using a stud chain when you are hand walking. I've had colts that I had to put on stall rest and all of them acted this way just from having to much energy from being cooped up. Expect that they will still be full of energy, but the chain gives you enough control if you need it and tells them what is too much with their behvior. You may have to get after them a time or two with it at first, but they usually figure it out really quickly. 

a little ace may help take the edge off.