the shock collor is the best thing you can do now.you are right , he should have started when they were about 3 to 4 mo. old.

most dogs would rather run a deer than a rabbit. the deer has a stronger sent.
if you don't get them off the deer running now , you will lose the game.

a friend of mine in Lousinna only runs rabbits and I get the ones that he can't break from running deer.some dogs just can't be broken.
He has been doing this for 35 to 40 years.( I like it when he can't stop one from running deer ; I get it) what ever you do ; do not shoot a deer in front of them.that will make a deer dog for life.

good luck in you training.