Partway up the tail on the top side of the tail is a sebaceous gland that is normally not active but in some dogs can get infected and sore often as a part of a reaction to allergies or flea bites.
As you have treated him for fleas (good choice!) what you might try with the sore area on the tail is cleaning it and treating it to see if that clears it up. You can use 3% hydrogen peroxide or betadine iodine solution to clean the area off. Then try some Neosporin or Triple antibitoic ointment on the scabby area. If you do that a couple times a day and see no change over the first 2-3 days of doing this then you should see a vet for more care. If it looks like it is healing keep the treatment up until its all healed.
The ointment won't hurt the dog if he licks it but it works better on the sore than the tongue so you can put on the ointment and then put some peanut butter or perhaps some baby food or Nutrical (a sticky supplement for dogs) on the roof of the dog's mouth to keep his tongue busy while the ointment soaks in some.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics