Several things, first I agree with everyone on here, as he grows he needs more space for excersise, is he getting that? Could be the issue.
Second, you stated that he used to be the sweetest thing. I often here this statement from people that always babied and pampered their calves, which I am not saying is bad, but they often let these calves learn bad habbits because they never really teach them manners and respect. In these instances the calf isnt necessarily acting out, he just thinks your another cow basically, you never taught him to respect you so now he is just growing up and part of what he is doing he may think is playing the other part is part of cattle maturing is natural bullying to determine who is boss, if you have a group of calves that have ALWAYS been together you wont see this as much because they just accept that the one that was always leader will always be leader. May be that he never truly respected you and is now challenging your authority.
Third, it is not uncommon for this to happen with steers, if he isnt flat out being mean, it could just be extra energy. As we get closer to show time we feed a higher energy feed because energy is stored as fat, feeding a hotter/higher energy feed often means animals with more energy which can be a little unruly.
Regardless it all is corrected the same, the tractor is pretty much inefective for all situations. The only thing I have ever had success with a tractor is one that just refuses to move, it does nothing for one that is trying to take off which it sounds like yours is doing. First you have to get his attention. Use your showstick, when he tries to take off pop him on the nose with it to get his attention. Also dont get frustrated and give up and quit working with him while he is acting up, this reinforces his bad behavior and lets him know that he is going to be left alone if he acts that way. Always keep working until he is doing something correctly before you quit for the session, this way you are reinforcing what you want him to do and not the bad behavior.