The best way to start a young squirrel dog is to find a woods/forest with plenty of squirrels and get your dog out there ALOT!
Don't ever shoot a squirrel out that your dog didn't tree.
When he trees, tie/leash him under the tree until you shoot the squirrel out.
This will teach the dog that you won't shoot a squirrel out until he has tree it and stayed-put under the tree.