good advise already just wanted to add what is going on with your pup, the primary socializing stage for a dog is 4 weeks to 16 weeks(4 months) at 16 weeks they go thru what is called a juvenile Fear period, where they actually feel, know and understand Fear. They will not go up to anything so easily or readily at 4 months old, IF they have not had Pleasant nice, good socializing before this fear period kicks in. When you try to socialize at this late stage with no previous stuff, then it is harder to get them to get over things and actually go up to things. The secret is like they say here, go slow, encourage, (praise when they do go forward) but don't push your pup into these bad situations. If at anytime time your puppy is scared- tries to run away, tuck their tail, attempts to bite or barks:
Do not try to soothe and pet them and say �it�s okay�. You will be praising and rewarding his scared behavior, and he will feel as though there is something he should be scared about, and it will make the situation worse.
Act happy and excited about the new thing, use treats and make it a game if possible.
If the situation is overwhelming, back away from the situation until the puppy feels and acts relaxed again.
Start from the new distance and build their confidence, act happy and excited, make it a game.
Gradually get closer to the situation.
Your goal is to get the puppy to feel comfortable and confident around the situations that they were scared about.
Hope this helps and get to a Positive reinforce class.