Hi mate, dont get disappointed, the more you lamp him the more he'll learn, also dont forget that he's probably not that fit yet neither as its only the start of the season, I've got a 20mth old and she's a bit the same, its frustrating but the more you do the sooner they'll learn.
My mate had the litter sister to my last dog, she was slow to catch, I bet she missed well over rabbits at first and sometimes she just didnt look interested, my mate was really frustrated with her and was ready to get rid, I convinced him to keep her and she turned out to be as good as any dog he had in the pa