Dog has crusty sores/pustules(pics)!! plz need advice

[rkmm3]

I have taken my dog to the vet, intially he had hot spots so i was given 40 pills of cephalexin 500mg along with itching meds, now after completing this and fixing that issue he has this sores around his butt right above his tail and some on his back legs nothing really towards the front. He is fine, plays fine. My vet gave me another 40 of the same pill along with itching meds. He said sometimes it takes 2-3 months of the antibolitcs to fix the issue. He did have and issue in the past regarding a skin infection. I know its hard to diagnose online. the first time he gave a different pill i think it was prednisone(big white pill) if im not mistaking and that basically saved his life but that time it was pretty severe and i had taken him to 3 different vets that didnt do any good.

Chocolate lab aussie shepherd mix. about 3 years old. mostly outside dog. but has been in since the issue. in FL

but now he has this.......and he is being treated with cephalexin.