Agility???

First congratulation on the new dog I bet he is fun.
 
Akc agility requires a dog be at least a year old  I believe every other agilty orginization requires a dog be 18 months old I probably wouldn't trial any dog till they where at least that old if not 2 because of the joint damage.
  You can absolutly start training now I would start with obedience because stays and attention are a very important part of agility. one thing you can do nowis teach hime to pay attention to your hands by feeding him his dinner from your hand switch back and forth beween hands there is a lot of signaling with your hands in agility for switching sides jumps so on. Agility is a lot of fun but more training than it appears. Find a class and have some fun.

I wasn't going to put him in any trials until he was about 2 and a half years old but I was wondering if I needed to startgetting him to try go through tunnels or see if he would do some small jumps.

be careful of jumping him.  Don't send him over anything higher than his elbows.  His growth plates are not completely done, yet.  One obstacle training exercise that is really good for young dogs is walking over a ladder on the ground.  They can't run, helter skelter over it.  They have to learn to step through it one opening at a time.  It teaches them to pay attention to what their rear legs are doing and where their body is. 

Also, he is just entering into adolescence and that is the most difficult time to keep his focus on you.  The best thing to work on now are exercises that will teach him to pay attention to you and follow your lead.  Make it fun! 

Why not try to find a class?  It would be a good socializing experience for him and it is always best to start obstacle training with an instructor on hand.  And it's fun for you!

Correction to the AKC comments:
 
As of September 1, 2006 Dogs will have to be 18 months old to compete in AKC Agility
 
The Dog walk will be raised to "Big dog" height permamently (no more small dog walk).  I apologize for not knowing the exact height of the dogwalk
 
The up contact of the dogwalk will be extended to account for the longer stride of the large breeds
 
The count on the table will change from starting over when a dog breaks position, to continuous
 
Now, moving on, congrats on your new English Shepherd
 
A dog needs to be trained in agility for AT LEAST a year before being entered in a trial.  A dog is NOT ready for agility unless he has a solid recall and can work under distraction.
 
My Strauss is 2 years of age, and we have been training in agility since April of this year.  He will not be trialing until LATE 2007 at his earliest.  I may choose to wait until 2008, simply because he is so immature at the age of 2 (mentally speaking), and is not, at this point in time, working consistently under distraction

If the OP has an English Shepherd, the dog won't be competing in AKC agility any time soon.  Look up rules for NADAC and USDAA.  Both require a dog to be 18 months before competing.