Suggest Treatment For Severe Ear Pain

Can I still give azithromycin antibiotic to my 2 year old son after 8 days?
The doctor prescribed it for my 2 year old for a cold virus but since he seemed to be getting better I thought it'd be ok if I didn't give it to him. It's been about 8 days since I picked it up from the pharmacy and today my son is complaining of an earache. He might have an ear infection and can't sleep well. I gave him motrin for the pain.

Is it ok if I give him the 200 mg (600 when mixed) azithromycin now with the prescribed directions of 1 tsp the first day and 1/2 tsp for 4 more days? or is it too late?

and if so, I'm just curious to know why exactly it can't be used after 10 days. Will it just not have any effect or is there an additional reason?
Thank you.