I agree with the others Abbie, don't risk little Reggie's development by over exercising until he is mature.
I used to do a fair bit of running in my younger days, and I never managed to get any of our spaniels to run with me, too busy with noses to the ground, and interesting sniffs to investigate.
William's no different it's hard enough to get him to keep up when walking, unless of course he's got his cocker mates to run with.
M.