Hi there, sounds like he may have allergies, this could have been to the fleas which I believe is quite common or could be lots of other things.
Allergies are rare in such young dogs but are getting more common. My Alfie has had allergies since he was about 4 months it took a long time for our vet to diagnose as he was so young but we took him to a doggie dermatologist and are waiting now to get full allergy testing done, my derm wouldn't do this until he was a year old and he just had his birthday on August 6th. Here are some pics when he was really bad.
Attachment 390680 
Right now you can help to manage the symptoms by feeding a good quality grain free hypoallergenic food. I buy baby gros and cut a whole for his bottom and up the front for him to pee he wears these at night and when we are at work to minimise the damage he can do to his skin. Keep his nails trimmed and put baby boogie socks on his hind legs, if he nibbles as well as scratching you may need to use a collar on him. Al of this will give his skin some rest and let the hair grow back.
Alfie is a much better happier dog now, with steroid, antihistamine, and medicated shampoo treatments. He has also had to have quite a few antibiotic treatments when he has kicked himself raw or broken his skin in seconds of scratching. Here he is now and with his homemade jammies.
