Welcome to DT! Your Rex is adorable.
I'd keep an eye on where he is licking to see if he is chewing on a dry spot with hair loss, etc. If so your vet can do a skin scraping when you take him for his next round of shots. If you want to clean the area off, just get some apple cider vinegar and dilute it with water...stick in a spray bottle and spray the spot and towel dry. I'd put about 1/4 cup vinegar in a large spray bottle and fill with water. You can do whole puppy actually to take any smells or mud he may get into. I use this as a quick winter bath for my guys. Not too much vinegar or you'll dry the coat out too much.
I like leather buckle collars and leather leads for this age group. Keep training short and fun. Do lots of play, games, use diversion onto a tug toy or a fluffy for biting episodes, get him out for some exercise and watch him like a hawk for potty breaks. You will be amazed how big this guy will be by 6-8 months of age. They grow big so fast. (Note: male Dobermans really don't mature until they are about 2 yrs old so be prepared for a ginormous puppy for a while mentally!)
(I also have a Rex, or Rexy, as we tend to call him. He's the old man of my group at age 9.) Enjoy your boy!
Rex is a cute looking fella! 
